Lake Texcoco (Spanish: Lago de Texcoco) was a natural lake formation within the Valley of Mexico. The Aztecs built the city of Tenochtitlan on an island in the lake. The Spaniards built Mexico City over Tenochtitlan. Efforts to control flooding led to most of the lake being drained, leaving a much smaller Lake Texcoco west of the city, surrounded by salt marsh.
The Valley of Mexico is a basin with an average elevation of 2,236 metres (7,336 ft) above mean sea level located in the southern highlands of Mexico's central altiplano. It formerly extended over a large portion in the southern half of the basin, where it was the largest of an interconnected chain of five major and several smaller lakes (the other main lakes being Xaltocan, Zumpango, Chalco and Xochimilco lakes). During periods of high water levels—typically after the May-to-October rainy seasons—the lakes were often joined as one body of water, at an average elevation of 2,242 metres (7,356 ft) above mean sea level. In the drier winter months the lake system tended to separate into individual bodies of water, a flow that was mitigated by the construction of dikes and causeways in the Late Postclassic period of Mesoamerican chronology. Lake Texcoco was the lowest-lying of all the lakes, and occupied the minimum elevation in the valley so that water ultimately drained towards it. The Valley of Mexico has been a closed basin since at least the Late Pliocene, and the lakes subsequently had no natural surface outflow, with the drainage basin forming an endorheic system.
The lake basin is now occupied by Mexico City, the capital of the present-day nation of Mexico.
In the Pleistocene era, the lake occupied an even greater area. There were several paleo-lakes that would connect with each other from time to time. At the north in the modern community of San Miguel Tocuila there is a great paleontological field, with a lot of pleistocenic fauna. The disarticulated remains of seven mammoths dated between 10,220 ± 75 and 12,615 ± 95 years (BP) were found, suggesting human presence.