Maybe you've seen one on the news: a giant yawning hole in the road, swallowing an unfortunate car that was in the wrong place when the Earth opened up without warning. Or maybe you were on vacation in Mexico and saw one of the country's beautiful cenotes, or water-filled pools, surrounded by verdant green. Both of these geological features are sinkholes, formations that are much more than holes in the ground. Though many are less than 100 feet (30 meters) deep, sinkholes can look like ponds, cover hundreds of miles or fit discreetly in your backyard [source: USGS].
Also called sinks, sinkholes owe much to water. A sinkhole usually forms by erosion caused by frequent exposure to water. It comes down to the type of rocks underlying the soil (as opposed to the soil above called the overburden). Most sinkholes occur in areas where the bedrock is formed from soft minerals and rocks like salt, gypsum, limestone, dolomite or others belonging to the evaporate or carbonate classes of rocks.
Sinkholes typically develop slowly as bedrock is whittled away by water turned acidic from absorbing carbon dioxide and interacting with plants. Rainwater obviously plays a role, but unseen water also matters. As the acidic water dissolves rock, it carves out conduits, or underground passages, for water. These conduits in turn help to develop underground basins known as recharge areas. Recharge areas contribute to the formation of sinkholes as water flowing to and from them and into the subsurface (the earth overhead) erodes bedrock. When water floods a developing sinkhole, some of the topsoil and other material can be caught in the conduits, further trapping water and limiting its ability to flow outward.
A lack of water can contribute to sinkholes, too. In some underground cavities, water may actually be holding up a thin overhang of earth. If that water level falls, the overhang has no support and collapses.
Sinkholes appear all over the United States. Florida, with its frequent rains and marshy terrain, is usually identified with sinkholes, but they're also prominent in Alabama, Kentucky, Missouri, Pennsylvania, Tennessee and Texas [source: USGS]. They generally develop in karst regions, a type of terrain known for soft bedrock [source: Southwest Florida Water Management District]. The U.S. Geological Service describes karsts as having many water elements, such as springs, underground streams, caves and, of course, sinkholes [source: USGS].
Next, we'll look at how different types of sinkholes form.