Earth’s surface is ever changing. Sinkholes swallow neighborhoods, river deltas slowly slide beneath the waves, and fertile fields lose elevation as farmers draw large amounts of water for irrigation from underlying aquifers. [T]hese phenomena are known as subsidence … Building heavy structures such as homes, skyscrapers, and even entire cities also can trigger subsidence, explains Tom Parsons, a geophysicist at the US Geological Survey in Moffett Field, CA. Recently, he used computer simulations to estimate the effect of San Francisco and its metropolitan area—where he and 7.75 million other people reside—on Earth’s crust.