The ground beneath our feet, our cars, our buildings, appears to be incredibly solid. But, rarely, that solid ground can simply open up without warning, dropping whatever was above into an unpredictably deep.hole. Sinkholes can be anywhere from a few feet wide and deep, to two thousand feet in diameter and depth. An undiscovered cavern or deep mine can collapse, allowing the ground above to crater, or a broken water main or heavy storm can erode a hole from below, until the surface becomes a thin shell that collapses at once. Communities built atop karst formations are very susceptible, where a layer of bedrock is water-soluble, like limestone, and natural processes can wear away caves and fissures, weakening support of the ground above. Gathered here are images of some of these sinkholes, both man-made and natural, around the world.