National Geographic defines sinkholes as "basically any collapsed or bowl-shaped feature that's formed when a void under the ground creates a depression into which everything around it drains."
© Christopher Furlong/Getty Images
A common geological occurrence, sinkholes may vary in size from a few feet across to being large enough to swallow an entire building.
© Christian Gooden/St. Louis Post-Dispatch/TNS via Getty Images
Sinkholes usually happen in "karst terrain," which are landmasses with a soluble bedrock, usually limestone and gypsum. Over time, the bedrock starts dissolving and creates pores and cracks. Ultimately, sinkholes start forming when the land above collapses completely or sinks down into cavities.
© Jim Young/Newscom/Reuters
Although it is a natural phenomenon, several human activities can lead to the formation of sinkholes. Drilling new wells, excessive extraction of groundwater, a collapsed mine or salt cavern and creating artificial ponds are some practices that may prompt sinkholes to form.
© Chris Wattie/Reuters
In urban settings, sewer collapses or changes in water-drainage patterns are believed to be common causes. Constructing parking lots and buildings disturbs the hydrologic regime of the groundwater, which leads to water being concentrated in an area rather than getting soaked in the ground naturally.
© Imaginechina/Corbis Nationalgeographic.com identifies two basic kinds of sinkholes: cover-subsidence and cover-collapse. The phenomenon is the same but the time they take to occur marks the difference. The former happens in sandier soils with a void underground. The soil gradually caves in the rock over time and the ground subsides. The latter usually occurs in clay since it holds soil together. Like cover-subsidence, soil leaks into a cave creating a void in the soil that moves upward. However, because we can't see it on the surface, it looks it like it happens suddenly, leading to incidents like bridge collapses.
© kenez/Getty Images
There has been a sudden increase in the number of sinkholes around the world, with most reported from the U.S state of Florida, whose terrain is classified as mostly karst. Other karst areas across the world are in countries like Mexico, Belize, Slovenia, Croatia, China, Russia and Italy.
© Steve Dipaola/AP Photo
Those that occur in natural environments are hard to predict and therefore not preventable. However, urban sinkholes can possibly be prevented by careful management of drainage and sewage systems.
© Owen Humphreys/PA Wire/Press Association
Sometimes there are telltale signs like cracks in the foundations of buildings, walls or on the ground outside. Also be on the lookout for skewed door frames, tilting fence posts or trees near a slumping terrain, exposed surfaces of trees, unexplained damage to vegetation, sudden appearance of ponds in dry places or a "chimney hole," a deep hole with steep sides.
© Pascal Guyot/AFP/Getty Images Some examples of sinkholes:
© Hiroshi Yamamura/EPA/Shutterstock
In Fukuoka, a large sinkhole cut off an avenue causing blackouts and disrupting the traffic on Nov. 8, 2016.
© Ulises Rodriguez/EPA/Shutterstock
A giant sinkhole swallowed at least 20 homes in Guatemala City in February 2007.
© Raminder Pal Singh/EPA-EFE/Shutterstock
Incessant rains caused a sinkhole in Amritsar on Sept. 24, 2018.
© Ginopress B V/EPA-EFE/Shutterstock
A blown water pipe caused a sinkhole in Wijchen on March 2, 2018.
© Massimo Percossi/EPA-EFE/Shutterstock
In Rome, a sinkhole swallowed many cars on Feb. 15, 2018.
In Uijeongbu, a sinkhole sucked in a vehicle on Sept. 5, 2018.
A natural sinkhole was seen in Sichuan on Feb. 23, 2013.