This is a huge problem, huge headache, the sinkhole. It can start off small and get bigger and bigger. "Sinkholes typically develop where construction crews have buried debris, such as tree branches, stumps, and construction materials. After a few years this buried debris decays. This leaves behind a large underground cavity hidden by a relatively thin surface layer of soil and grass. The soil layer eventually caves in, causing a sunken area or hole that can seem bottomless." Read more about this here. And obviously, if you're going to be selling your home anytime soon, this needs to be addressed before it gets worse.