The ground beneath our feet, our highways, our cities, appears to be very solid. But, on rare occasions, that solid ground can simply open up without warning, dropping whatever it was supporting into an unpredictably deep hole. An abandoned mine or undiscovered cavern can cave in, or a broken water main or heavy storm can erode from below, until the surface becomes a thin shell that collapses all at once. Sinkholes can be anywhere from a few feet wide and deep, to two thousand feet in diameter and depth. Collected below are images of some of these sinkholes, both man-made and natural, around the world.