Valley of Mexico

The Valley of Mexico is a highlands plateau in central Mexico roughly coterminous with the present-day Distrito Federal and the eastern half of the State of 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. The ancient Aztec term Anahuac (Land Between the Waters) and the phrase Basin of Mexico are both used at times to refer to the Valley of Mexico. The Basin of Mexico became a well known site that epitomized the scene of early Classic Mesoamerican cultural development as well.

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 Valley of Mexico can be subdivided into four basins, but the largest and most-studied is the area which contains Mexico City. This section of the valley in particular is colloquially referred to as the "Valley of Mexico". The valley has a minimum altitude of 2,200 metres (7,200 ft) above sea level and is surrounded by mountains and volcanoes that reach elevations of over 5,000 metres (16,000 ft). It is an enclosed valley with no natural outlet for water to flow and 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. Seismic activity is frequent here, and the valley is considered 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 BCE to 800 CE), the Toltec Empire (10th to 13th century) and the Aztec Empire (1325 to 1521). 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, the Spaniards rebuilt the largest and most dominant city here, Tenochtitlan, renaming it Mexico City. The valley used to contain five lakes called Lake Zumpango, Lake Xaltoca, Lake Xochimilco, Lake Chalco, and the largest, Texcoco covering about 1,500 square kilometres (580 sq mi) of the valley floor, but as the Spaniards expanded Mexico City, they began to drain the lakes' waters to control flooding. 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 ground water 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.