Most of the states prone to this strange phenomenon are in the South.

By Valerie Fraser Luesse
September 27, 2017

Most of the states prone to this strange phenomenon are in the South.

Orlando Sentinel/Getty Images

Hurricane Irma put Florida sinkholes back in the spotlight, but the Sunshine State has long been dealing with them, as have Texas, Alabama, Kentucky, Tennessee, Missouri, and Pennsylvania.

The New York Times reported that Land O’Lakes, Florida, where an enormous sinkhole opened up before the hurricane, is part of an area near Tampa known as “Sinkhole Alley.” Dr. Manoj Chopra of the University of Central Florida told an NBC affiliate in Orlando that the sudden increase in water levels caused by Irma produced great pressure, which can contribute to rapidly developing sinkholes.

So what is a sinkhole?

We found the simplest definition at nationalgeographic.com: “A sinkhole is 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.”

In other words, it’s a hole. That sinks. Into the ground. And while we can't predict exactly where they'll happen, we do know a bit about what causes sinkholes.

How do sinkholes form?

According to the U.S. Geological Survey, sinkholes start in areas where there’s no natural drainage, and rainwater continually seeps down into the rock below ground. If that rock happens to be of a type that groundwater can dissolve—like limestone, salt beds, or carbonate rocks—water cuts its way through the rock and begins eroding soil underground.

Some sinkholes happen slowly and appear as a depression that gets bigger and deeper over time. But the most dangerous ones are called “cover-collapse” sinkholes. They happen when the surface soil contains lots of clay, which can hold the top layer together long enough for groundwater to erode a huge cave beneath it. Eventually, the surface wears so thin that it can no longer hold any applied weight. Imagine crossing a city street and stepping on a manhole cover that’s made of paper. The surface material isn’t strong to suspend you above the cavern below, so down you go.

Do all sinkholes form naturally?

Not exactly. Humans sometimes help them along. A collapsed mine or salt cavern can cause a sinkhole; so can underground water pipes that break or urban development that doesn’t have a sound drainage plan.

More sinkhole facts from livescience.com:

The National Corvette Museum sinkhole, which collapsed in 2014 in Bowling Green, Kentucky, (while the museum was closed, fortunately) swallowed 8 cars. [Restored cars from the event became part of the museum’s Corvette Cave In: The Skydome Sinkhole Experience.]

Water-filled sinkholes called “blue holes,” which occur in caves or the ocean, are meccas for divers. Popular among them is Dean’s Blue Hole in the Bahamas—663 feet deep.

No one knows where the world’s largest sinkhole is, but China has a good shot at the title, thanks to the so-called Heavenly Pit, which is 2,170 feet deep.

Among the warning signs of a potential sinkhole collapse are leaning fence posts or trees near ground that is noticeably slumping; exposed surfaces of trees, foundations, etc., that were previously covered; the appearance of circular patches of wilting vegetation; water suddenly gathering in small ponds where the soil was previously dry; and the appearance of a “chimney hole”—a deep vertical hole with steep sides.

WATCH: Southern Places You're Probably Mispronouncing

Florida has its share of them— Ichetucknee, Weeki Wachee . . . Just please tell us you haven't been roaming around Disney World talking about the great hotel deal you got in KISS-uh-me, bless your heart.

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