Valley of Mexico
The Valley of Mexico (Spanish: Valle de México; Nahuatl languages: Anahuac, lit. 'Land Between the Waters'), sometimes also called the Basin of Mexico, is a large natural basin on the highland plateau of central Mexico. Surrounded by mountains and volcanoes, the valley was the historical center of several pre-Columbian civilizations including Teotihuacan, the Toltec, and the Aztec Empire, and today is almost entirely occupied by Mexico City, the capital of the modern nation of Mexico.
A natural endorheic basin, the Valley of Mexico historically contained five shallow, interconnected lakes called Lake Zumpango, Lake Xaltocan (Nahuatl languages: Xāltocān), Lake Xochimilco, Lake Chalco, and the largest, Lake Texcoco, which at their greatest extent together covered about 1,500 square kilometers (580 sq mi) of the valley floor. When Spanish explorers first arrived in the Valley of Mexico in 1519, 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. By the 20th century the lakes had almost entirely disappeared and the municipalities of Mexico City eventually grew to occupy the reclaimed land of the former lakebed.
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. It is approximately 125 km (78 miles) long and 75 km (47 miles) wide with a NNE-SSW orientation, covering a total surface area of approximately 9,600 km2 (3,700 sq mi). The Valley of Mexico can be subdivided into four basins, but the largest and most studied is the area that contains Mexico City. This subsection of the larger valley in particular is often 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 the 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 Spanish conquest, by 1900 it had again grown to 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 and is one of the most populous urban areas in the world.
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 airborne pollutants in the valley. Overextraction of groundwater has also 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.