thumb|The Valley of Mexico at the time of the [[Spanish conquest of the Aztec Empire|Spanish conquest in 1519]]
thumb|View of the Valley of Mexico from the neighborhood of San Bernabé Ocotepec, 2022
The Valley of Mexico (; ), 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 (), Lake Xochimilco, Lake Chalco, and the largest, Lake Texcoco, which at their greatest extent together covered about of the valley floor. 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 long and wide with a NNE-SSW orientation, covering a total surface area of approximately . 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 above sea level and is surrounded by mountains and volcanoes that reach elevations of over . 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.
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). 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.
History of human habitation
First human habitation
The Valley of Mexico attracted prehistoric humans because the region was rich in biodiversity and had the capacity of growing substantial crops. 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 site located near the town of Texcoco in Mexico State. 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. 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. In the early 8th century, with the rise of the Toltec empire, Teotihuacan ceased to be a major urban centre and the population shifted to Tollan or Tula on the northern front of the Valley of Mexico.
The largest and most dominant city at the time of the Spanish conquest was Tenochtitlan. It was founded by the Mexica (Aztecs) on a small island in the western part of Lake Texcoco in 1325, and was extended with the use of chinampas, human-made extensions of agricultural land into the southern lake system, to increase productive agricultural land, covering about .
Air pollution
Mexico City is vulnerable to severe air pollution problems due to its altitude, its being surrounded by mountains and the winds patterns of the area. The altitude, with its low oxygen levels, makes for poor combustion of fossil fuels leading to unsafe levels of nitrogen oxides, hydrocarbons, and carbon monoxide. All the other lakes flowed toward the lower Lake Texcoco, which was saline due to evaporation. Warmer temperatures had increased evaporation and reduced rainfall in the area so that the lakes’ waters were shallow at about deep as early as the Tlapacoya culture, around 10,000 BC.
The arrival of the Spanish and subsequent efforts to drain the area for flood control was a major infrastructure project, called the desagüe, which was pursued throughout the entire colonial period.
The idea of opening drainage canals first came about after a flood of the colonial city in 1555. The first canal was begun in 1605 to drain the waters of Lake Zumpango north through Huehuetoca which would also divert waters from the Cuautitlán River away from the lakes and toward the Tula River. This project was undertaken by Enrico Martínez and he devoted 25 years of his life to it. He did succeed in building a canal in this area, calling it Nochistongo, leading waters to the Tula Valley, but the drainage was not sufficient to avoid the Great Flood of 1629 in the city. Another canal, which would be dubbed the "Grand Canal" was built parallel to the Nochistongo one ending in Tequixquiac. The Grand Canal consists of one main canal, which measures in diameter and long. The drainage project was continued after independence, with three secondary canals, built between 1856 and 1867. During the presidency of Porfirio Díaz (r. 1876–1911) drainage again became a priority. Díaz completed it officially in 1894, although work continued thereafter.
As a result, another tunnel, called the Emisor Central, was built to carry wastewater. Although it is considered the most important pipe in the country, it has been damaged by overuse and wall corrosion of the diameter pipe.
Because of this, another new drainage project is planned that will cost US$1.3 billion. The project includes new pumping stations, a new drainage tunnel and repairs to the current system of pipes and tunnels to clear blockages and patch leaks.
Over-pumping of groundwater in the 20th century has hastened the disappearance of the lakes. The old lake beds are almost all paved
Desiccation has had a major environmental impact on the Valley of Mexico.
Drinking water and sinking lands
thumb|right|upright|The [[Independence Angel statue: street level has sunk below the bottom of the statue.]]
Historically, Mexico City's potable water supply came via aqueduct from the mountain springs on the valley sides like that in Chapultepec as most of the water in Lake Texcoco was saline. No water from these rivers is allowed to sink into the ground to recharge the aquifer. While the rivers and streams that flow down from the mountain peaks still begin the way they always have, their passage through the shantytowns lacking city sanitation schemes that surround Mexico City turns them into open combined sewers. Therefore, their final stages are frequently culverted or added to the existing major culverted rivers to keep this water from contaminating the aquifer.