Lake Texcoco
Lake Xochimilco was originally a part of an even larger Lake Texcoco during the last glacial period. Between 12,000 and 6,000 years ago, the climate in central Mexico warmed and the snowmelt that once fed Lake Texcoco virtually disappeared. This caused the lake to drop hundreds of feet over next several thousand years. By 2,000 years ago Xochimilco became a bay in the southern portion of Texcoco. Then around the 13th or 14th century the Aztecs built large causeways effectively creating a new lake.