Today’s Wonder of the Day was inspired by Susan. Susan Wonders, “How do sinkholes form?” Thanks for WONDERing with us, Susan!

Every once in a while, you'll see a news story about a sinkhole that suddenly opens up and causes quite a stir. Although it may seem like these are rare oddities, sinkholes are actually quite common all over the world.

Sinkholes go by many names around the world, including sinks, shake holes, swallow holes, swallets, dolines, and cenotes. Although they may be called by different names, they have similar causes.

Sinkholes are depressions or holes in the Earth's surface caused by natural karst processes. Karst processes occur when water and chemicals dissolve carbonate rocks, such as limestone, to form sinkholes and caves.

For example, one common scenario involves the gradual erosion of bedrock by ground water that percolates down through cracks in the bedrock. As the rock erodes and weakens, spaces and caverns develop underground.

The land above the developing spaces usually remains intact until the rock underneath can no longer support it. When the underground spaces grow large enough, a sudden collapse of the surface land above can occur.

This is what happens when a sinkhole "suddenly" opens up. Of course, the underlying processes have been going on for some time.

Sinkholes can also be caused by human factors. For example, man-made mines that are no longer used occasionally cause collapses. Water and sewer pipes that break can also sometimes lead to the formation of sinkholes if they create underground flows that speed the process of erosion.

Sinkholes range in size from 3 feet to over 2,000 feet (.9 meters to 609 meters) — both in width and depth. The largest known sinkhole in the world is in China. Called the Xiaozhai tiankeng, it is almost 2,200 feet (670 meters) deep!

Sinkholes that form in coral reefs or islands can often be very deep and filled with water. They're known as blue holes, because of the deep blue color of the water in these holes. Blue holes often become popular spots for divers to explore.

Not all holes that suddenly open up in the ground are sinkholes, though. In May 2010, a huge hole opened up suddenly in Guatemala City, swallowing a three-story building and a house. Although news reports called this hole a "sinkhole," it was actually something scientists call a "piping pseudokarst."

Because there is no carbonate rock under Guatemala City, the hole couldn't be caused by karst processes and thus technically couldn't be defined as a sinkhole. Instead, the hole was caused by the collapse of large cavities that had developed over time in the thick volcanic ash deposits that exist under Guatemala City.

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