A sinkhole is a large depression in the Earth's surface, often shaped like a well or a funnel, that can be up to several miles in diameter. Sinkholes are most common in limestone regions. Underground layers of limestone can be dissolved by groundwater or by seepage from above-ground streams. A crack in underground limestone is often the first step in the formation of a sinkhole. A sinkhole may also be caused by a dramatic event, such as the collapse of a cave roof.