Mexico City

Mexico City was first built on islands of Lake Texcoco by the Aztecs, who devised innovative agriculture and damming practices to thrive upon the salty lake. The Spanish colonial regime began draining the lakes of Mexico Valley in the 17th century to mitigate flooding and create farmland—but salty soil proved difficult to cultivate, and as the groundwater drained away, the lakebed sank and the land gradually dried. The threat of water shortage now hangs over Mexico City, the largest urban center in North America (population 8.9M), while dust clouds from the dessicated valley contribute to massive levels of air pollution.