Resting on a fickle lake bed in the Valley of Mexico, Mexico City has been facing the possibility of going under for centuries. However, according to geologists, there are parts of this massive metropolis that are sinking by as much as eight inches per year. Because of a lack of sufficient drainage, the city was heavily prone to flooding from groundwater flowing down from the surrounding mountains, leading to the construction of a vast underground drainage system. But over-extraction of groundwater from the Texcoco lake bed during the 20th century has caused the city's clay foundation to crumble, forcing the metropolis to sink deeper into the ground. And as the city sinks, so does the drainage system, forcing waste water to reverse its course and head back into the city. According to AZCentral.com, Mexico City's drainage capacity has decreased by 30 percent since 1975.
There are several projects underway to keep this sinking city afloat, including a 23-foot-wide ($1.1 billion) Eastern Drainage Tunnel that will deposit waste water about 40 miles north of the city. The tunnel should be completed by 2012. Work is also being done to help save the architectural gems -- like the popular Metropolitan Cathedral -- found in the Historic District, which are one of the city's major tourist draws. But despite best efforts, engineers say that there is no way to keep Mexico City from sinking.