Sinking Mexico City

ABC News -   The fact that Mexico City is sinking is not new. NASA says it has been documented the changes for more than a century.  According to a 1995 report by the National Academies of Sciences, Engineering, and Medicine, the city was already sinking roughly two inches per year by the late 1800s. By the 1950s, that number jumped to 18 inches, the report found.

The first finding was reported by engineer Roberto Gayol in 1925, who pointed to a large canal and tunnel built to drain water out of the city's waterlogged ground as the potential cause.  Scientists now point to a more direct culprit — decades of draining the ancient lakebed aquifer that the city was built on.

As water is pumped out, the ground above it compacts and stays that way, according to a study published by the American Geophysical Union. Think of wet clay that gets squeezed flat and hardens in place.