Valley of Mexico

The Valley of Mexico (Spanish: Valle de México; Nahuatl languages: Anahuac, lit. 'Land Between the Waters'), sometimes also called Basin of Mexico, is a highlands plateau in central Mexico. Surrounded by mountains and volcanoes, the Valley of Mexico was a centre for several pre-Columbian civilizations including Teotihuacan, the Toltec, and the Aztec Empire. The valley used to contain five interconnected lakes called Lake Zumpango, Lake Xaltocan (Nahuatl languages: Xāltocān), Lake Xochimilco, Lake Chalco and the largest, Lake Texcoco, covering about 1,500 square kilometers (580 sq mi) of the valley floor. When the Spaniards arrived in the Valley of Mexico, it had one of the highest population concentrations in the world with about one million people. After the conquest of the Aztec Empire, the Spaniards rebuilt the largest and most dominant city, Mēxihco Tenōchtitlan, renaming it Ciudad de México (Mexico City) and over time began to drain the lakes' waters to control flooding.

The Valley of Mexico is located in the Trans-Mexican Volcanic Belt. The valley contains most of the Mexico City metropolitan area, as well as parts of the State of Mexico, Hidalgo, Tlaxcala, and Puebla. The Basin of Mexico covers approximately 9,600 km² (3,700 sq mi) in the NNE-SSW direction with length to width dimensions of approximately 125 km (78 miles) to 75 km (47 miles) The Valley of Mexico can be subdivided into four basins, but the largest and most-studied is the area that contains Mexico City. This section of the valley in particular is colloquially referred to as the "Valley of Mexico". The valley has a minimum elevation of 2,200 meters (7,200 ft) above sea level and is surrounded by mountains and volcanoes that reach elevations of over 5,000 meters (16,000 ft). It is an enclosed valley with no natural outlet for water to flow to the sea although there is a gap to the north where there is a high mesa but no high mountain peaks. Within this vulnerable watershed all the native fishes were extinct by the end of the 20th century. Hydrologically, the valley has three features. The first feature is the lakebeds of five now-extinct lakes, which are located in the southernmost and largest of the four sub-basins. The other two features are piedmont, and the mountainsides that collect the precipitation that eventually flows to the lake area. These last two are found in all four of the sub-basins of the valley. Today, the Valley drains through a series of artificial canals to the Tula River, and eventually the Páruco River and the Gulf of Mexico. Seismic activity is frequent here, and the valley is an earthquake-prone zone.

The valley has been inhabited for at least 12,000 years, attracting humans with its mild climate (average temperatures between 12 and 15 °C, or 54 and 59 °F), abundant game and ability to support large-scale agriculture. Civilizations that have arisen in this area include the Teotihuacan (800 BC to 800 AD) the Toltec Empire (10th to 13th century) and the Aztec Empire (1325 to 1521). Although violence and disease significantly lowered the population of the valley after the Conquest, by 1900 it was again over one million people. The 20th and 21st centuries have seen an explosion of population in the valley along with the growth of industry. Since 1900, the population has doubled every fifteen years. Today, around 21 million people live in the Mexico City Metropolitan Area which extends throughout almost all of the valley into the states of Mexico and Hidalgo.

The growth of a major urban industrial centre in an enclosed basin has created significant air and water quality issues for the valley. Wind patterns and thermal inversions trap contaminants in the valley. Over-extraction of groundwater has caused new flooding problems for the city as it sinks below the historic lake floor. This causes stress on the valley's drainage system, requiring new tunnels and canals to be built.

The Valley of Mexico attracted prehistoric humans because the region was rich in biodiversity and had the capacity of growing substantial crops. Generally speaking, humans in Mesoamerica, including central Mexico, began to leave a hunter-gatherer existence in favor of agriculture sometime between the end of the Pleistocene epoch and the beginning of the Holocene. The oldest known human settlement in the Valley of Mexico is located in Tlapacoya, located on what was the edge of Lake Chalco in the southeast corner of the valley in contemporary Mexico State. There is reliable archeological evidence to suggest that the site dates as far back as 12,000 BC. After 10,000 BC, the number of artifacts found increases significantly. There are also other early sites such as those in Tepexpan, Los Reyes Acozac, San Bartolo Atepehuacan, Chimalhuacán and Los Reyes La Paz but they remain undated. Human remains and artifacts such as obsidian blades have been found at the Tlapacoya site that has been dated as far back as 20,000 BC, when the valley was semi-arid and contained species like camels, bison and horses that could be hunted by man. However, the precise dating of these artifacts has been disputed.

Giant Columbian mammoths once populated the area, and the valley contains the most extensive mammoth kill sites in Mexico. Most of the sites are located on what were the shores of Lake Texcoco in the north of the Federal District and the adjacent municipalities of Mexico State such as in Santa Isabel Ixtapan, Los Reyes Acozac, Tepexpan and Tlanepantla. Mammoth bones are still occasionally found in farmland here. They have been discovered in many parts of the Federal District itself, particularly during the construction of the city's Metro lines and in the neighborhoods of Del Valle in the center, Lindavista to the center-north and Coyoacán in the south of the city. The symbol for Line 4's Talisman station of the Mexico City Metro is a mammoth, due to the fact that so many bones were uncovered during its construction. However, the richest site for mammoth remains in the valley is at the Paleontological Museum in Tocuila, a 45-hectare (110-acre) site located near the town of Texcoco in Mexico State. Although there is some evidence around the old lakeshores that the first populations here survived by hunting, gathering, and possibly by scavenging, evidence from this time period is scarce.

Tlatilco was a large pre-Columbian village and culture in the Valley of Mexico situated near the modern-day town of the same name in the Mexican Federal District. It was one of the first significant population centers to arise in the valley, flourishing on the western shore of Lake Texcoco during the Middle Pre-Classic period, between 1200 BC and 200 BC. It was originally classified as a necropolis when it was first excavated, but it was determined that the many burials there were under houses of which nothing remains. It was then classified as a major chiefdom center. The Tlatilcans were an agricultural people growing beans, amaranth, squash and chili peppers, reaching their peak from 1000 to 700 BC.

The next-oldest confirmed civilization is in the far south of the valley and is called Cuicuilco. This archaeological site is located where Avenida Insurgentes Sur crosses the Anillo Periférico in the Tlalpan borough of the city. The old settlement once extended far beyond the boundaries of the current site, but it is buried under lava from one of the volcanic eruptions that led to its demise, and much of the modern city is built over this lava. The settlement was located where an old river delta used to form in the valley with waters from Mount Zacatépetl located in what is now the Tlalpan Forest. Cuicuilco was believed to have reached city status by 1200 BC and began to decline around 100 BC - AD 150. However, even though the ceremonial pyramid was abandoned, the site remained a location to leave offerings up to AD 400, although lava from the nearby Xitle volcano completely covered it.