The ground beneath our feet, our highways, and our cities appears to be very sturdy. But, on rare occasions, that solid ground can simply open up without warning, dropping whatever it was supporting into an unpredictably deep hole.
An undiscovered cavern or abandoned mine might collapse, or a broken water main or heavy storm might cause erosion, until the surface becomes a thin shell that drops away all at once.
Sinkholes can be anywhere from a few feet wide and deep, to 2,000 feet in diameter and depth. Collected below are images of some of these sinkholes, both man-made and natural, around the world.