Researchers at Baylor University are working on adapting lithophanes—an ancient artistic medium—for modern efforts to make scientific images accessible and understandable via touch. Lithophanes, which are thin, detailed engravings made from translucent materials, are thought to have originated in China as early as the seventh century and rose in popularity in Europe in the 1800s. The new lithophanes from the Baylor researchers are the first known examples of their kind and are intended for use as tactile learning tools for blind and visually impaired individuals. The new lithophanes can enable sightless or low-vision individuals to visualize microscopic and nanoscopic structures—for instance, in the subjects of anatomy and chemistry—with a greater level of detail and retainability than previously possible—and even at the same "resolution" as that of sighted individuals. See also: Learning; Visual impairment

Using traditional welding methods, rocket engine nozzles and combustion chambers are some of the most difficult parts to fabricate, requiring up to a year to produce a single part. Now, however, the U.S. National Aeronautics and Space Administration (NASA) is using additive manufacturing technology, also known as 3D printing, to additively build those parts, layer upon layer. This technology differs completely from traditional manufacturing methods that manufacture destructively by taking away material until a final geometry is realized. 3D printing has allowed NASA to produce rocket engine nozzles with integrated cooling channels in 30 days by way of an additive process called blown powder directed energy deposition (BP-DED). See also: Nozzle; Rocket propulsion; 3D printing

3D printing, also known as stereo modeling or rapid prototyping, starts with a computer-aided design (CAD) [digital drawing] file of an object. The CAD file is then converted to a 3D printer file, which represents the object in layers and directs the printer to deposit layer upon layer of material to build an object from bottom to top. See also: Computer-aided design and manufacturing

Cosmochemists have discovered the oldest solid material known to exist on Earth. The material, stardust, consists of silicon carbide grains measuring around one micrometer in size. The grains date back approximately 7 billion years, or about 2.5 billion years before the Sun and solar system formed. Aged stars nearing the end of their life cycles originally forged the grains, which then were captured inside a meteorite that formed around 4.6 billion years ago, eventually crashing into Australia in 1969. Analyzing the dust will help scientists study previous stellar generations in our galaxy. See also: Milky Way Galaxy; Star; Sun

On July 20, 1969, the bulkily space-suited figure of astronaut Neil Armstrong descended the ladder of Apollo 11’s lunar lander, planted his boot in the Moon's gray dust, and declared: "That's one small step for a man, one giant leap for mankind." This first visit to Earth's natural satellite—the culmination of NASA's Apollo 11 mission—was a seminal moment in the history of human space exploration (Fig. 1). See also: Moon; Space flight

On April 29, 2015, the U.S. Geological Survey (USGS) reported five earthquakes in Oklahoma of magnitude 2.5 or greater—that is, tremors strong enough to be felt. The most powerful of these was a M4.1 earthquake 21 km (13 mi) west of the city of Perry in north-central Oklahoma. Based on recent evidence reported by the USGS, these earthquakes were almost certainly induced by human activity as a consequence of the injection of wastewater from oil and gas production into deep disposal wells. See also: Earthquake; Oil and gas field exploitation; Well

On April 25, 2015 at 11:56 a.m local time, a magnitude 7.8 earthquake struck in the district of Lamjung, approximately 80 km northwest of Kathmandu, Nepal. The earthquake, now known as the Gorkha quake, occurred as the result of thrust faulting [15.0 km (9.3 mi) in depth] in the region where the India plate is sinking (subducting) beneath the Eurasia plate, according to the United States Geological Survey (USGS). Damage to buildings was extensive and over 8700 people were killed. See also: Earthquake; Fault and fault structures; Subduction zones

A magnitude 5.1 nuclear test was detected 376 km (234 mi) NE of Pyongyang, North Korea (DPRK) on January 6, 2015 at 10:30 a.m. local time (01:30 UTC), according to the U.S. Geological Survey (USGS). The event was picked up at seismic stations around the world. The Comprehensive Nuclear-Test-Ban Treaty Organisation (CTBTO) said that the explosion occurred in roughly the same area as North Korea’s previous nuclear test on February 12, 2013. See also: Nuclear explosion; Nuclear testing; Seismographic instrumentation

The Ebola virus comprises a group of pathogenic agents that cause severe and deadly hemorrhagic fevers in humans and other primates. During 2014 and 2015, an Ebola virus epidemic in West Africa—predominantly, the countries of Guinea, Liberia, and Sierra Leone—killed more than 11,300 people; it was the worst outbreak of this disease in recorded human history. At that time, the World Health Organization (WHO) declared the Ebola outbreak to be a Public Health Emergency of International Concern (PHEIC), and massive efforts were undertaken to contain the spread of the deadly disease. By early 2016, the affected countries were declared free of Ebola virus transmission, and the epidemic was considered to have ended. However, since then, isolated outbreaks have occurred sporadically, mostly as a result of the virus persisting in survivors after recovery. Most troublesome, though, the Ebola virus was detected in August 2018 in the Democratic Republic of the Congo (DRC) in Central Africa and has since expanded dramatically. It is now the second-largest Ebola outbreak on record. As of June 2019, more than 2000 Ebola cases have been detected, with more than 1350 resultant deaths. See also: Africa; Ebola virus; Ebola virus outbreak in 2014–2015; Exotic viral diseases; Infectious disease; Virus

The United Nations has proclaimed 2019 the International Year of the Periodic Table of Chemical Elements (IYPT2019). The same year also marks the 150th anniversary of the publication of Russian chemist Dmitri Mendeleev’s periodic table, containing 63 elements, in which the known elements were arranged horizontally by their atomic weight and vertically in groups of elements with similar "periodic" properties. The present periodic table, containing 118 elements, is completely filled. It includes element 101 named mendelevium (Md). According the United Nations, the purpose of this commemoration of the periodic table is, “to enhance global awareness of and education in the basic sciences.” A number of IYPT2019 special events have been planned. See also: Atomic mass; Mendelevium; Periodic table

Wildfires are a natural phenomenon in many ecosystems, including forests along the U.S. Pacific West Coast, as natural fires serve to rejuvenate ecological communities. However, from mid-August through mid-September 2020, forests in California, Oregon, and Washington State suffered through unprecedented wildfires caused by extreme weather events that can be attributed to climate change. By September 15, 2020, in Washington state, 700,000 acres (280,000 hectares) had burned; in Oregon, about 1 million acres (405,000 hectares) had burned; and in California, the worst-hit state, more than 7700 fires had burned 3 million acres (1.2 million hectares), damaged or destroyed more than 5000 structures, and killed at least 35 people. The single largest fire to date was the California Complex fire in Northern California, which was still raging on September 15 after it had already burned about 800,000 acres (320,000 hectares). These epic blazes have created some of the worst air pollution in the world from Seattle, Washington, to Southern California, with haze reaching as far east as New York City. See also: Air pollution; Extreme weather events; Forest fire; Global climate change; Wildfire impact on air quality

Active traffic management (ATM) uses advanced technologies (computing, communication, and electronics) and traffic management centers to improve roadway traffic flow. Adaptive traffic signal control is an ATM solution for reducing traffic congestion through intersection signal (traffic light) optimization using real-time data. The essential components of the control system are roadside traffic sensors, a central computer (control center), traffic-signal controllers at the intersections, and a fiber-optic or wireless communication system. In recent years, Los Angeles and New York City have made significant investments in upgrading their traffic signals to adaptive control. See also: Active traffic management; Data communications; Highway engineering; Optimal control theory; Optimization; Traffic-control systems; Ubiquitous transportation network sensors

The Joshua tree (Yucca brevifolia) is a remarkable evergreen, treelike plant found only in the southwestern United States. It was named by Mormon settlers in the early 1800s. Upon seeing the limbs of this plant branching upwards to heaven, the Mormon travelers were reminded of the biblical prophet Joshua, who raised his arms in prayer for guidance to the Promised Land. The Joshua tree is the largest member of the Yucca genus of monocotyledonous trees and shrubs, which is placed in the agave family (Agavaceae; order Asparagales) or the lily family (Liliaceae; order Liliales), depending on the taxonomic system being used for classification. The height of the Joshua tree can reach 15 to 40 feet (4.6 to 12.2 meters) and its diameter ranges from 1 to 3 feet (0.3 to 0.9 meters). Life spans of 150 to 200 years have been documented for this resilient species, with some unverified ages of 500 to 1000 years being estimated. See also: Asparagales; Liliales; Monocotyledons; Tree

There is a growing body of evidence that many of the thousands of prescription and over-the-counter medications ultimately make their way into water bodies and drinking-water supplies. Typical wastewater treatment plants may not be effective at removing pharmaceuticals. This is because there are no regulations limiting the release of these substances in the environment and, as a result, the needed treatment technology may not have been investigated or implemented. Knowing the best treatment practices for removing pharmaceuticals from wastewater is important for protecting drinking-water supplies as well as organisms in marine and aquatic environments because many pharmaceuticals are designed to be effective at low concentrations. A new study shows that removal of pharmaceuticals from wastewater is possible, even at very low concentrations, depending on the specific treatment process, according to researchers reporting in Environmental Science: Water Research & Technology (January 2020). See also: Environmental toxicology; Freshwater ecosystem; Marine ecology; Water pollution; Water resources; Water treatment

As social animals, humans tend to form groups that can range in size from a few individual members to entire nation-states. Historically and through the present day, human groups frequently come into conflict. A noted feature of human intergroup conflict is the proclivity of ingroup members—that is, members recognized as being part of a particular group—to inflict harm upon those considered to be non-members who are part of an outgroup. A new study may help explain the motivation for this hostility. The study found that aggression directed at rival group members increases activity in certain brain regions that are associated with feelings of reward and pleasure. Investigating this potential link further could deepen our understanding of the origins of intergroup aggressiveness and suggest interventions for curbing group-based violence. See also: Aggression; Brain; Motivation; Sociobiology

Microplastics are raining down on national parks and other protected areas in the western United States at the rate of about 132 pieces of microplastic per square meter every day, according to a report in the journal Science (June 2020). Microplastics are plastic particles measuring less than five millimeters (approximately 0.2 inches) in length. The reported precipitation rate is similar to dumping about 300 million pulverized plastic water bottles (about 1000 tons) over the studied areas each year. Where do these particles come from? Sources mostly include fragments from virgin plastic pellets, polymer textile fibers, microbeads from personal care products and spray paints, as well as environmental degradation of waste plastic into smaller and smaller pieces over time. See also: Manufactured fiber; Paint and coatings; Plastic waste pollution; Polymer; Textile

The benefits of outdoor physical activity (exercise) outweigh the potential harm caused by air pollution except in the most highly polluted cities, according to epidemiological studies reported in the journal Preventive Medicine (February 2016). Exercise provides a number of health benefits, including reduced risks of cardiovascular disease, type 2 diabetes, and some cancers. Cycling and walking, for example, offer not just health benefits but also environmental advantages as pollution-free means of transportation. See also: Air pollution; Cancer (medicine); Epidemiology; Heart disorders; Sports medicine; Type 2 diabetes

A research group at the European Organization for Nuclear Research (CERN) has performed the first-ever high-precision measurement of the gravitational weight of antimatter, a rare substance that appears identical to ordinary matter in all properties except for electric charge. As described in a recent paper, antimatter experiences the same amount of gravitational attraction on Earth as ordinary matter, at least up to the precision limits available in ALPHA-g, the experiment that produced the results. The finding offers new insights into the physical nature of antimatter. See also: Antimatter; Earth's gravity field; Electric charge; Matter; Weight (physics)

Advances in aluminum-air battery technology might allow electrically powered vehicles to become more realistic and competitive automotive options for many drivers.

The species diversity and ultimate survival of amphibians, one of the major groupings of vertebrate animals, face tremendous challenges today for reasons that are still not entirely understood. Since the 1980s, the pace of amphibian extinction has been alarming, and almost 200 identified species have become extinct. About one-third of all amphibian species have seen dramatic declines in populations, and are now considered to be endangered or threatened with extinction. See also: Biodiversity; Endangered species; Extinction (biology); Extinction of species

Collectively, members of the class Amphibia are facing massive reductions in total numbers, and the overall biodiversity of amphibians across the globe is under threat. Scientists have determined that global climate change and habitat destruction are two chief factors responsible for some of the decreases observed in amphibian populations. However, another prime culprit has been discovered—specifically, chytridiomycosis. Chytridiomycosis is a disease caused by the waterborne fungal pathogen Batrachochytrium dendrobatidis (abbreviated as Bd). Since the 1980s, scientists have linked the chytrid fungus to the significant loss of much amphibian biodiversity in many locations around the world. More than 500 amphibian species have suffered total number losses as a result of chytridiomycosis, with at least 90 of these species becoming extinct. In addition, 124 species have suffered a 90% reduction in their populations, endangering their continued existence. Although all amphibian species are thought to be susceptible to chytridiomycosis, frogs and toads (order Anura) have been affected to the greatest degree. See also: Amphibia; Anura; Biodiversity; Endangered species; Extinction; Fungi; Global climate change; Mycology; Origins of modern amphibians

The ancient Egyptian civilization flourished along the banks of the Nile River, which supplied the waters that ensured a prosperous and lengthy agricultural vitality over the course of thousands of years. On the strength provided by this rich agricultural productivity, the Egyptian civilization was able to develop into one of the foremost and notable ancient empires, excelling in architecture (including the use of the ramp and the lever in construction), science, medicine (including the art of mummification), mathematics, writing (hieroglyphic script), art, engineering, stone masonry, irrigation techniques, and commerce. See also: Africa; Agricultural science (animal); Agricultural science (plant); Agriculture; Anthropology; Archeology; Architectural engineering; Inclined plane; Irrigation (agriculture); Lever; Masonry; Mathematics; Medicine; River; River engineering; Science; Scientific methods; Stone and stone products

Sea-level rise is caused by adding mass (water) and heat to the ocean. The Antarctic ice sheet represents a potentially large source of water. Knowing the speed and direction (velocity) at which the Antarctic ice sheet is flowing to the ocean is important for assessing current and future sea-level rise. Scientists from the University of California, Irvine, and the NASA Jet Propulsion Laboratory have published the most precise map ever of Antarctic ice velocity in the journal Geophysical Research Letters (July 2019). The new map will be useful for mass-balance and ice-thickness studies of the Antarctic ice sheet as well as for modeling climate and sea-level rise. Compared to earlier maps, the new map is 10 times more accurate in terms of ice-flow direction and speed and shows ice movement over 80 percent of Antarctica versus 20 percent shown in past maps. See also: Antarctica; Climate modeling; Glaciology; Global climate change; Sea-level rise

In 1987, the Montreal Protocol, the international treaty banning chemicals that deplete the ozone layer, was adopted. Thirty years later, NASA scientists reported in the journal Geophysical Research Letters (January 2018) satellite measurements showing decreased stratospheric ozone depletion as a result of declining levels of ozone-depleting chlorine from the breakdown of chlorofluorocarbons (CFCs)—that is, chemicals that were used as solvents, refrigerants, and aerosol-can propellants. The ozone hole is an area of severely depleted ozone in the stratosphere over Antacrtica, appearing at the beginning of Southern Hemisphere spring (August–October). From 2005 to 2016, instruments aboard the NASA Aura satellite measured chlorine and ozone levels in the Antarctic ozone hole. As compared to measurements taken in 2005, the measurements showed about a 20 percent decline in ozone depletion during the Antarctic winter. This is the first study to confirm that the ozone hole has begun healing and that the recovery can be attributed to the implementation of the Montreal Protocol. See also: Antarctica; Halogenated hydrocarbon; Ozone; Stratosphere; Stratospheric ozone

Researchers have developed polymeric materials that are able to kill multidrug-resistant bacteria without inducing drug resistance or toxic side effects, according to a report in Nature Communications (March 2018). The polymers, known as guanidinium-functionalized polycarbonates, contain a polycarbonate backbone to which a hydrophobic spacer group carrying a guanidinium C(NH₂)₃⁺ group is attached. The positively charged guanidinium group enables the polymer to be prepared as a water-soluble salt as well as to bind to the negatively charged cell walls of bacteria. After attachment, the polymer penetrates and crosses the cell membrane and then precipitates the proteins and genes in the cytoplasm, effectively killing the bacteria. Antibiotic resistance in bacteria is a serious problem worldwide, with the World Health Organization in January 2018 reporting 500,000 people with suspected antibiotic-resistant infections in 22 countries. See also: Antibiotic; Antibiotic resistance; Antimicrobial agents; Drug resistance; Infectious disease; Polymer

It is estimated that 1 gram of soil can be inhabited by up to 10⁹ microorganisms and approximately 60,000 bacterial species. Moreover, soil harbors a vast reservoir of antimicrobial agents, and soil-dwelling bacteria have played a key role in the introduction of antibiotics to treat infectious diseases. Because these resilient bacteria not only produce antibiotics but also are exposed to other antibiotics produced by surrounding strains in the soil, they have developed diverse mechanisms to survive the toxic antimicrobial compounds created around them. Importantly, these mechanisms of robust resistance to numerous classes of antibiotics often resemble the mechanisms of resistance identified in clinical pathogens, including those that infect humans. Thus, scientists are attempting to find possible correlations between antibiotic resistance in soil bacteria and in infectious agents in humans. If correlations can be found, investigators might be able to predict future signs of clinical resistance to certain antibiotics, providing clinicians with methods to circumvent any potential resistance that may emerge. See also: Antibiotic; Antimicrobial agents; Antimicrobial resistance; Bacteria; Clinical microbiology; Drug resistance; Infectious disease; Medical bacteriology; Microbiology; Pathogen; Soil; Soil microbiology

Antisense drugs are gene-based molecules that inhibit the synthesis of proteins (including proteins that cause specific diseases) by binding to the ribonucleic acids (RNAs) responsible for their formation. Specifically, these drugs are single-stranded short polymers of RNA or deoxyribonucleic acid (DNA), termed oligonucleotides, designed to contain part of the noncoding strand of messenger RNA (mRNA), which is a molecule involved in translating DNA into protein. Antisense medications are therefore capable of hybridizing with and inactivating the mRNA, preventing the associated gene from producing the unwanted protein. With their anticancer, antiviral, and anti-inflammatory therapeutic capacities, these drugs have been applied in the treatment of various genetic disorders and infections, including diabetes, rheumatoid arthritis, cytomegalovirus retinitis (a virally caused form of blindness that occurs often in AIDS patients), asthma, hypercholesterolemia (a genetic derangement of fat metabolism characterized by very high levels of cholesterol in the blood), and numerous cancers. Research is also being conducted on patients suffering from Parkinson's disease and Huntington's disease to determine whether antisense therapy can mitigate the effects of these conditions. See also: Biotechnology; Deoxyribonucleic acid (DNA); Disease; Gene; Genetic engineering; Oligonucleotide; Protein; Ribonucleic acid (RNA)

A powerful magnitude 7.8 earthquake struck Ecuador on April 16, 2016 at 6:58 p.m. local time, causing extensive damage to buildings and infrastructure as well as hundreds of fatalities and thousands of injuries. The epicenter was located 52.0 km (32.3 mi) west of Rosa Zarate. The earthquake occurred as the result of thrust faulting 19.2 km (11.9 mi) in depth near the boundary of the Nazca and Pacific plates, according to the U.S. Geological Survey (USGS). See also: Earthquake; Fault and fault structures; Subduction zones

To bolster food security in a warming world, engineers are developing a coating that could better protect seeds, such as beans, from drought in semiarid regions. The coating mimics a mucilage-based hydrogel produced naturally by some seeds, such as basil and chia. The hydrogel traps moisture, regulates nutrient retention, and creates an environment around the seed which promotes the growth of beneficial microorganisms. The artificial coating is accordingly tailored to help protect seeds during germination and early seedling growth. In semiarid regions, water stress caused by a lack of available moisture during these sensitive periods of a plant's development is the highest cause of crop loss worldwide. This water stress on agricultural crops in drought-prone, marginal lands is expected to substantially increase in coming decades because of global climate change. See also: Agricultural engineering; Drought; Farm crops; Global climate change; Mucilage; Seed

Soil harbors a vast reservoir of antimicrobial agents. In fact, approximately 80% of all clinically implemented antibiotics are derived from soil-dwelling bacteria. However, many bacteria have evolved methods to evade the effects of various antibiotics and thus have acquired resistance to these chemical substances. Today, the increasing resistance of many common disease-causing bacteria to antibiotics is a global health crisis. Therefore, scientists are seeking to develop new antibiotics that can overcome the resistance capabilities of numerous pathogenic bacteria. Arylomycins are considered to be among the most promising of these new compounds. See also: Antibiotic; Antibiotic resistance; Antibiotic resistance in soil bacteria; Antimicrobial agents; Bacteria; Infectious disease; Medical bacteriology; Microbiology; Pathogen; Public health; Soil; Soil microbiology

The Asian longhorned tick (Haemaphysalis longicornis), also known as the bush tick, is a troublesome parasitic species and is an important vector of disease agents. (Note that vectors are capable of biologically transferring a pathogen from one organism to another.) It also is becoming a problematic invasive species. Specifically, it is invading areas located far from its native region in East and Central Asia and is spreading within the United States. In particular, this arachnid is remarkable because female members can reproduce asexually without mating. Thus, scientists fear that the species will spread rapidly in locations where it has already invaded, thereby transmitting numerous diseases that are potentially harmful to humans and other animals. See also: Acari; Disease; Invasive species; Ixodides; Parasitology; Pathogen; Tick virus diseases; Zoonoses

Although asphalt (also known as bitumen) is a naturally occurring substance found in deposits within the earth, it is also a by-product of petroleum processing. Almost all the bitumen used for asphalt pavement and roofs comes from petroleum refining and consists of hydrocarbon compounds that remain at the upper end (600°C or 1100°F) of the vacuum-distillation process. Because asphalt cannot be vacuum distilled, it has long been assumed that installed asphalt pavement and roofs do not emit hydrocarbon pollutants. Yet, according to a new report in the journal Science Advances (September 2020), asphalt pavements and roofs do, in fact, emit significant quantities of hydrocarbons, including polycyclic aromatic hydrocarbons (PAHs), particularly in environmental conditions found on hot and sunny days. This is problematic, because some PAHs are mutagens, teratogens, carcinogens, or endocrine disruptors and therefore harmful to human health and the environment. Hydrocarbons may also act as precursors to air pollutants, because hydrocarbons can react with nitrogen oxides (NO_x) and sunlight to form ozone, a major constituent of smog. See also: Air pollution; Aromatic hydrocarbon; Asphalt and asphaltite; Environmental toxicology; Nitrogen oxides; Ozone; Pavement; Petroleum; Petroleum processing and refining; Petroleum products; Roof construction; Smog

In September 2020, researchers answered a longstanding question about why planetary nebulae display such a rich variety of shapes. Winds from aging stars produce planetary nebulae as the winds eject the stars' outer layers, shrouding the stars in stellar material. Views through early telescopes made this ejected material appear rounded and planet-like. As telescope technology improved, however, astronomers came to realize that planetary nebulae can look like flowers and butterflies, among other varied, asymmetrical shapes. Those shapes confounded astronomers, given their assumption that stellar winds are spherical in nature, similar to stars themselves, and should thus produce only rounded nebulae. The September 2020 study revealed, however, that these winds actually blow asymmetrically. Furthermore, as winds throw material off the star unevenly, stellar companions—such as other stars and planets—can scramble the material, rather like a spoon stirring creamer in hot coffee. Overall, these effects combine to produce the observed diversity of nebular forms. See also: Nebula; Planet; Planetary nebula; Star; Telescope; Wind

On August 5, 2018 at 6:46 p.m. local time, a magnitude 6.9 earthquake struck in Pulau Lombok, Indonesia. The earthquake occurred as the result of thrust faulting at a depth of 10.5 km (6.5 mi) in northern Lombok, according to the United States Geological Survey (USGS). A tsunami warning was issued, but was lifted after only small waves were recorded. Early reports stated that 98 people were killed and more than 200 injured, as the earthquake caused many buildings to collapse. Some structures that collapsed had already been damaged one week earlier (June 29, 2018) when a magnitude 6.4 earthquake struck Lombok, killing 17 people and injuring more than 160. Indonesia is located in a very seismically active zone, in which magnitude 5.0 or greater earthquakes were recorded on 17 of 31 days in July 2018. See also: Earthquake; Earthquake engineering; Fault and fault structures; Seismology; Tsunami

Wildfires, also termed bushfires or forest fires, are currently devastating large swaths of land in Australia. Detrimental ecologic consequences of these uncontrolled combustions of forest fuels and vegetation are numerous. One of the most serious issues pertains to the survival of the koala, which is a small marsupial (pouch-bearing mammal) found only on that continent. Conservationists and scientists are concerned that the hundreds of fires presently raging in Australia, which were brought about by hotter and drier conditions exacerbated by anthropogenic climate change, may cause significant enough reductions in the population of koalas to threaten the continued existence of this species. See also: Australia; Forest fire; Global climate change; Global warming; Koala; Marsupialia

The Aztecs were an indigenous group of Mesoamerican tribal peoples whose influence dominated that region between the fourteenth and early sixteenth centuries. They predominantly spoke a native language called Nahuatl, and they lived a nomadic existence prior to the twelfth century. See also: Agricultural science (plant); Agriculture; Bean; Corn; Domestication (anthropology); North America

The Babylonian civilization flourished from approximately 1900 to 539 BC in the central and southern regions of Mesopotamia (present-day Iraq), predominantly between the Euphrates and Tigris rivers. Before the rise of the Babylonians, the area was inhabited by the Sumerians in southeast Mesopotamia and the Akkadians in northwest Mesopotamia. Through a series of expansions and conquests undertaken by various dynastic kings, these two cultures were incorporated into the Babylonian empire, which eventually established control over many other kingdoms from ancient Persia to Syria and Palestine. Along with this political and geographical dominance, the cities of the Babylonian empire became rich centers of learning, especially in the areas of astronomy, astrology (including the division of the night sky into a zodiac of constellations), mathematics, and medicine. The most important city was Babylon, which was situated 88 kilometers (55 miles) southwest of modern-day Baghdad. Once famous for its impressive architectural marvels, including massive walls, ziggurats (pyramid-shaped temples built on platforms), and the Hanging Gardens of Babylon (considered to be one of the Seven Wonders of the Ancient World), few remains of this ancient city can be found today. In addition, it is likely that the biblical story of the Tower of Babel was based on an ancient Babylonian structure. See also: Anthropology; Archeoastronomy; Archeology; Architectural engineering; Astronomy; Mathematics; Medicine; Science; Scientific methods; Zodiac

Recycling or reuse is in most instances environmentally preferable to disposal. Reuse of coal tar as an asphalt-pavement sealant for driveways and parking lots, however, has turned out to be an exception to this rule of thumb. See also: Asphalt and asphaltite; Pavement

Bats are members of the order Chiroptera, which is the second-largest order of living mammals. The geographic distribution of these flying mammals is tremendous as well, ranging from the limit of trees in the Northern Hemisphere to the southern tips of Africa, South America, and New Zealand. Within such a wide distribution, bats frequently encounter human populations. However, close contact between bats and humans is problematic with regard to disease ecology and epidemiology because bats are natural reservoirs (primary hosts) or intermediate hosts for numerous zoonotic pathogens—that is, infectious disease agents that are transmitted from animals to humans. Specifically, bats harbor more than 60 pathogenic viruses that can infect humans, including Ebola, Marburg, Nipah, Hendra, and rabies viruses. Bats also harbor various coronaviruses, including those responsible for severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), and coronavirus disease 2019 (COVID-19). See also: Chiroptera; Coronavirus; Disease ecology; Epidemiology; Exotic viral diseases; Infectious disease; Mammalia; Pathogen; Rabies; Virus; Zoonoses

The American beech (Fagus grandifolia) is native to forest regions east of the Mississippi River and is a key tree species in the United States. In particular, it is very abundant from Ohio to the New England states. The tree is also found in the southernmost portions of eastern Canada, especially southern Ontario. Since 2012, though, American beech trees have been distressed by a lethal leaf disease—termed beech leaf disease—that has spread rapidly from Ohio into the other aforementioned geographical areas. Most cases of the disease have been detected in forest regions, but landscaped locations also have been affected. The external symptoms of the disease include striped banding and shriveling of tree leaves, aborted budding, reduced production of leaves, and premature leaf drop. More importantly, beech trees with beech leaf disease often die within a relatively short span of time after being inflicted by the disease—within 2 to 3 years for saplings and within 6 years for larger and older trees. See also: Beech; Forest; Forestry; Leaf; Tree; Tree diseases

The dance language of honeybees (Apis mellifera) is a unique form of animal behavior and communication that was discovered by the zoologist Karl von Frisch (1886–1982) in the 1920s, which he continued to study through the 1960s and for which he was awarded a Nobel Prize in 1973. When foraging worker honeybees return to the hive with pollen or nectar, they perform dance movements to recruit other bees (follower bees) to visit a food source. The specifics of these movements convey information about the distance and direction from the hive to the food. This type of communication is highly unusual: Nonhuman animal communication typically concerns conditions and events immediately surrounding the instigator, whereas the honeybee dance language transmits information about remote events [scientists have documented bee dances about food sources up to 15 kilometers (9.3 miles) away from hives]. See also: Animal communication; Beekeeping; Ethology; Hymenoptera; Pollen; Pollination; Social insects

Tardigrades, also known as water bears, are hardy eight-legged microorganisms ranging in length from 0.5 millimeters to 1.5 millimeters. Well known in the animal kingdom for their resilience in the face of extreme environments, tardigrades have survived in boiling water, deep sea trenches, and even space. Although the threshold of harsh conditions that tardigrades can withstand has been heavily studied, less understood is the biological mechanisms behind this unique tardigrade tolerance. See also: Astrobiology; Deep-sea trench; Space; Space biology; Tardigrada

Biocontrol, or biological control, is the natural or applied regulation of populations of pest organisms—especially insects—through the role or use of natural enemies. In other words, biocontrol uses living organisms to reduce and eliminate pest abundance and damage. The agents through which biocontrol is accomplished are varied. For example, biocontrol agents can be consumers of pests; thus, herbivores are used to reduce weeds, whereas predators or parasites are employed to diminish the number of insect and other animal pests. In addition, pathogens and competitors that interact with pests are often utilized in biocontrol. Typically, agricultural and forest pests are the major targets of biocontrol. Notably, biocontrol operates as an alternative to the use of pesticides that may be harmful to the environment. See also: Agricultural science (animal); Agricultural science (plant); Agricultural soil and crop practices; Agriculture; Biological insect control; Forest; Forestry; Insecta; Parasitology; Pathogen; Predator-prey interactions; Weeds

Biodegradable metals, also known as bioresorbable or bioabsorbable metals, are compatible with human tissues and degrade to nontoxic by-products. The most promising candidates for use as orthopedic and cardiovascular implants are alloys of magnesium, which biodegrade in 6–15 months, and alloys of iron, which biodegrade in 12–36 months. Both types of alloy degrade by corrosion—the oxidation and dissolution of the metals. See also: Alloy; Corrosion; Iron; Iron alloys; Magnesium; Magnesium alloys

For over fifty years, heat stabilizers, light stabilizers, and antioxidants have been added to polymers to maintain their material properties by preventing their degradation. This chemical adjustment is necessary to extend the life of products such as plastic pipes, paints and coatings, and automotive parts. More recently, additives with the opposite purpose have become available: They are designed to enhance the biodegradation of hydrocarbon polymers used in common products such as plastic bags. Biodegradation is the breakdown of organic compounds by microorganisms or other biological means to their inorganic mineral constituents, such as carbon dioxide (CO₂). Biodegradation additives would seem to be an elegant solution to the mounting environmental problem of plastic waste—except that recent research indicates they do not work. See also: Antioxidant; Biodegradation; Microbial ecology; Polymer; Polyolefin resins; Stabilizer (chemistry)

Many questions concerning the sustainable production of biofuels have emerged over recent years in terms of land and water use, pollution from fertilizer and pest-control chemicals, greenhouse-gas production, net energy production, and whether enhanced land-use and climate benefits would result from simply planting trees and eschewing biofuels altogether. According to researchers reporting in the Proceedings of the National Academy of Sciences (September 2020), the answer depends, in part, on optimizing land-use policies and bioenergy production systems. Researchers have concluded that sustainable biofuels could make an important contribution toward reducing greenhouse-gas emissions and stabilizing the global climate if we produce bioethanol from biomass sources, such as perennial grasses, that are grown without affecting the carbon already stored in the ecosystem. See also: Biomass; Ethyl alcohol; Global climate change; Greenhouse effect; Land-use planning; Reforestation

Humans have used dried bovine dung (manure) as a fuel source since prehistoric times. In India, for example, many people burn cow or buffalo dung for cooking fuel. This bioresource provides a low-cost fuel as well as an efficient means of waste disposal. However, in rural India, the burning of dung is simultaneously a source of hazardous indoor and outdoor air pollution, resulting from the emission of fine particulates that are 2.5 micrometers or less in diameter (PM_2.5). Such combustion byproducts are considered the most dangerous to human health, because PM_2.5 can be inhaled deep into the lungs, impairing lung function as well as and affecting other organs, such as the heart and brain, or causing cancer. See also: Air pollution; Indoor air pollution

Biologic drugs, also known as biologics or biologicals, are produced using living organisms. Various classic biologics, such as the medical human insulin taken by diabetics, have been made by biotechnology processes for some decades. What is relatively new are therapeutic biologics to treat autoimmune diseases and cancer, such as the trade-named monoclonal antibody drugs Remicade (infliximab) and Rituxan (rituximab) used to treat autoimmune diseases. See also: Antibody; Autoimmunity; Biologicals; Biotechnology; Molecular biology; Monoclonal antibodies

Although cats are the number one lethal threat to birds, collisions with building glass are not far behind. The number of birds killed each year by glass collisions worldwide is estimated to be in the billons. In the United States, the U.S. Fish & Wildlife Service estimates that between 400 million and one billion birds die each year from collisions with buildings. In contrast, an annual average of only 250,000 bird deaths result from collisions with land-based wind turbines. See also: Aves; Building; Glass; Wind power

Researchers recently discovered a black hole with a mass estimated at 3.3 times the mass of the Sun, which is significantly less than the lowest-mass black holes previously detected with approximately 5 times the Sun's mass. The new finding opens the door to the possibility that a sizable population of similarly low-mass versions of black holes has thus far escaped detection. Finding more members of this hypothetical population will help scientists better understand the origin and evolution of black holes, which are among the most extreme and enigmatic objects in the universe. See also: Black hole; Mass; Sun; Universe