Sinkholes occur when an underground void collapses, bringing the layers of earth and rock above it down with the collapse. The surface above the sinkhole suddenly drops, plunging anything in the area into a deep hole in the ground. Depending on the nature of the sinkhole, the bottom may be filled with water, and the sides often collapse inward and bury anything or anyone that falls in.
One common cause of sinkhole formation is the dissolution of minerals or weaker stone by groundwater. As water carries away particles of the material underground, it leaves a growing hole in the earth. Eventually, this becomes too large to support itself, resulting in a rapid collapse. Sinkholes can also occur when the water table drops suddenly, removing some of the pressure that was holding up the soil above and emptying underground caves and aquifers.
Artificial sinkholes also exist, created when underground mines or other structures collapse. A sinkhole can also occur when surface construction results in more pressure than underground materials can support, crushing weak stone and leading to a general collapse of the surrounding terrain.
The world's largest sinkhole is Xiaozhai Tiankeng in China, which measures 2,050 feet across by 1,760 feet long, and has a depth of at least 1,675 feet.