Lake Texcoco was one of the great natural lakes of the Valley of Mexico and is known as the place of the Aztec island city Tenochtitlan. Today, much of the land occupied by Mexico City was once underwater, and Lake Texcoco has lost more than 95% of its historic expanse. The lake disappeared and re-formed at least 10 times in the last 30,000 years. The Spanish conquered Tenochtitlán in 1521 and built their new colonial capital, Mexico City, on its ruins. Over the following centuries, the Spanish drained Lake Texcoco because flooding was becoming a major problem for the city. The Spanish sought to expand Mexico City past its island borders into the lakebed of Lake Texcoco. To do so, the Desagüe drained the surrounding water, and Mexico City expanded into the lakebed and the land past original borders. Today, all that is left of Lake Texcoco is some marshland near the international airport on the east side of the city, and some canals on the adjoining lake to the south, Xochimilco, have also survived. The modern Texcoco Lake has a high concentration of salts, and its waters are evaporated for their processing. After the cancellation of the Mexico City Texcoco Airport, the government initiated a major restoration project of a significant part of the lake in the form of the Lake Texcoco Ecological Park, 14,000 hectares of public space and ecological restoration.