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.

sinkhole
The Yucatan Peninsula is dotted with sinkholes like this one.

Daisetta Sinkhole
Some parts of the United States are very susceptible to sinkholes. In May 2008, a large sinkhole formed in Daisetta, Texas, a suburb of Houston. The sinkhole formed when an underground mound of rock salt collapsed. The sinkhole swallowed several cars, oil drilling equipment, and oil tanks. In one day, the Daisetta sinkhole had grown to 200 meters (656 feet) in diameter and 75 meters (246 feet) deep. Within a couple weeks a 23-meter (7-foot) deep lake had formed in the sinkhole, and a 2-meter (7-foot) alligator had taken up residence in the waters.

abundant
Adjective

in large amounts.

alligator
Noun

reptile native to the southeast United States and parts of China.

Noun

an underground layer of rock or earth which holds groundwater.

cave
Noun

underground chamber that opens to the surface. Cave entrances can be on land or in water.

cenote
Noun

natural sinkhole or reservoir where groundwater is available.

Noun

waterway between two relatively close land masses.

debris
Noun

remains of something broken or destroyed; waste, or garbage.

dissolve
Verb

to break up or disintegrate.

erode
Verb

to wear away.

freshwater
Noun

water that is not salty.

joint
Noun

crack in a rock where water can collect.

Noun

the geographic features of a region.

limestone
Noun

type of sedimentary rock mostly made of calcium carbonate from shells and skeletons of marine organisms.

Noun

an area within a larger city or town where people live and interact with one another.

prevalent
Adjective

common or widespread.

prone
Adjective

vulnerable or tending to act in a certain way.

rainfall
Noun

amount of precipitation that falls in a specific area during a specific time.

residence
Noun

home or place where a person lives.

road
Noun

path, usually paved, for vehicles to travel.

rock
Noun

natural substance composed of solid mineral matter.

rock salt
Noun

natural mineral form of salt (sodium chloride.) Also called halite.

Noun

hole formed in a rock or other solid material by the weight or movement of water.

suburb
Noun

geographic area, mostly residential, just outside the borders of an urban area.

susceptible
Adjective

able to be influenced to behave a certain way.

underworld
Noun

mythical or legendary place for the souls of the dead.