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.
Sinkholes are common where the rock below the land surface is limestone, carbonate rock, salt beds, or rocks that can naturally be dissolved by groundwater circulating through them. As the rock dissolves, spaces and caverns develop underground.
According to the U.S. Geological Survey, there are three common types of sinkholes: 1) dissolution, 2) cover-collapse, and 3) cover-subsidence.
A sinkhole is a closed natural depression in the ground surface caused by removal of material below the ground and either collapse or gradual subsidence of the surface into the resulting void.
Signs of a sinkhole on the property include exposure of previously business items, such as fence posts and foundations, vegetation that wilts and dies as the sinkhole draws away essential water, and a circular pattern of ground cracks.
Many engineers prefer the graded-filter technique, in which the hole is filled with a layer of boulders, then a layer of smaller rocks, and, finally, a layer of gravel. This fills the hole, more or less, while permitting water to drain through the area.
Jim Stevenson, a former Florida Park Service chief naturalist, says any hard objects that disappeared into the hole, such as chairs and tables, will sit and decay in the limestone cavern below.
The top layer of a cover-collapse sinkhole is usually a soft overburden, made of soil with a lot of clay in it. Over time, a small cavern forms underneath that soil. Opportunistic sediment takes advantage of the newly free real estate and starts spilling into the cavern -- a process known as spalling.
An undiscovered cavern or abandoned mine might collapse, or a broken water main or heavy storm might cause erosion, until the surface becomes a thin shell that drops away all at once. Sinkholes can be anywhere from a few feet wide and deep, to 2,000 feet in diameter and depth.
The deepest blue hole in the world at 300.89 meters (987 feet) deep is in the South China Sea and is named the Dragon Hole, or Longdong. The second deepest blue hole in the world with underwater entrance at 202 metres (663 ft) is Dean's Blue Hole, located in a bay west of Clarence Town on Long Island, Bahamas.
Solution sinkholes have sides that vary from gentle slopes to almost vertical, while their shapes include saucer-like hollows, cones, cylindrical potholes and shafts. Streams or rivers may enter a sinkhole and disappear underground. This type of sinkhole is often called a swallow hole.
"The main trigger for sinkholes is water," says Dr. Edmonds. "In 90% of sinkhole cases, water saturating the ground is the main trigger, known as Karst processes." "Sinkholes happen when a layer of rock underneath the ground is dissolved by water.
Any change to the hydrologic system (putting more water in or taking it out) causes the system to become at least temporarily unstable and can lead to sinkholes. Sinkholes can result from seasonal changes in the groundwater table, freeze and thaw of the ground, and extremes in precipitation (drought vs heavy rain).
A circular hole typically forms and grows over a period of minutes to hours. Slumping of the sediments along the sides of the sinkhole may take approximately a day's time to stop. Erosion of the edge of the sinkhole may continue for several days, and heavy rainfall can prolong the stabilization.
Sink holes can continuing growing for a surprisingly long time. This could be years or even decades. It all depends on how long the ceiling of the hole can support the weight of the sediment above.
They tend to occur most often in places where water can dissolve the bedrock (especially limestone) below the surface, causing overlying rocks to collapse. Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania are most sinkhole-prone.
But one thing is for certain: measuring 660m deep, with a volume of 130 million cubic metres, China's Xiaoxhai Tiankeng is both the deepest and largest sinkhole in the world.
The ground starts to buckle, and all of a sudden, you find yourself freefalling into a black void. Sinkhole accidents are relatively rare, but that doesn't matter to someone who's lost their home or became injured from falling into a sinkhole.
But one of the warning signs of sinkholes is an Earthy switch to the scent of rain. You also want to look for cloudiness in well water, or even debris leaking into your well. If your backyard begins to smell more like a garden than usual, you may have a sinkhole.