The land below many U.S. cities is sinking. Known as land subsidence, this natural and man-made phenomenon can have a direct effect on home values and put critical infrastructure such as railroads and airports in danger. Luckily, some land subsidence can be reversed. "It's not cheap and it's not easy. But we have tools in our toolbox that can implement that and see the results in … months or years," said Manoochehr Shirzaei, professor of geophysics and remote sensing at Virginia Tech.