A sinkhole is a hole in the ground that forms when water dissolves surface rock. Often, this surface rock is limestone, which is easily eroded, or worn away, by the movement of water.

In a landscape where limestone sits underneath the soil, water from rainfall collects in cracks in the stone. These cracks are called joints. Slowly, as the limestone dissolves and is carried away, the joints widen until the ground above them becomes unstable and collapses. The collapse often happens very suddenly and without very much warning. Water collects in these collapsed sections, forming sinkholes.

Sinkholes also form when the roofs of caves collapse. Sinkholes are often funnel-shaped, with the wide end open at the surface and the narrow end at the bottom of the pool.

Sinkholes vary from shallow holes about 1 meter (3 feet) deep, to pits more than 50 meters (165 feet) deep. Water can drain through a sinkhole into an underground channel or a cave. When mud or debris plugs one of these underground caves, it fills with water to become a lake or a pond.

Sinkholes occur naturally, especially where there is abundant rainfall, and the rock beneath the surface soil is limestone. For instance, a cenote (pronounced "seh-NOH-tay") is a type of sinkhole that forms when the roof of an underground cave collapses, exposing the water to the surface. Cenotes are very common on the Yucatan Peninsula of Mexico. There are more than 2,000 cenotes on the Yucatan, and they are a main source of fresh water for people there. Ancient Mayans believed cenotes were passageways to the underworld.

People can create sinkholes when building roads, aquifers, or other types of construction. Altering land in these ways can weaken the underlying rock and make it more susceptible to sinkholes. Sinkholes can open up in the middle of busy streets or in neighborhoods, especially during heavy rainfall.

The land surrounding the Dead Sea in the Middle East is prone to sinkholes because of the prevalence of rock salt, which is easily dissolved by water. Tourists who are unaware of sinkholes and even scientists studying sinkholes have been injured by falling into them.