Sinkholes: The Hole Truth

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CBS Web Copy

If you're a fan of science fiction B-movies from the 1950s, sinkholes make perfect sense. They seem almost as improbable as alien body snatchers. The Earth suddenly opens up, with no warning and no escape, and swallows whatever's on the surface -- cars, homes, entire neighborhoods.

But sinkholes, of course, are all too real worldwide, in places like China, Brazil and Guatemala. Florida has so many of them, it has been called "Swiss cheese covered by soil."

In 1981, a sinkhole in Winter Park, Fla., grew longer than a football field. One resident described a sound like "giant beavers chewing." And just last month, a sinkhole in Chicago ate up three cars.

Of course, the sinkhole horror story most of us know best is that one back in February in Seffner, Florida, near Tampa, where a sinkhole 20 feet wide and 60 feet deep opened in the middle of the night.

His body was never found -- the fourth known sinkhole fatality in Florida.

"About anywhere you live in Florida, there is potential for a sinkhole to occur," said state geologist John Arthur, an expert on sinkholes.

Florida's surface has close to 70,000 likely sinkholes; at least 3,400 of them reported since the 1950s.

A sinkhole is a naturally-occurring feature that forms when a cavity in the subsurface collapses. That cavity forms from the natural dissolving of rock.

What was a hole in the ground becomes a hole at land surface.

Florida's bedrock, mostly limestone, lies below layers of soil sand and clay. That bedrock is porous and over time can erode and become unstable, forming what geologists call "karst." From time to time, natural or man-made changes in the water table collapse this karst and create sinkholes.

Those collapses can happen in minutes, and take months to settle.

All states but Hawaii have at least some karst. "That's true," said Arthur. "If you look at a map of karst for the nation, there are little corners of even South Dakota that have the potential for some type of karst activity."

Though some states have a greater risk of sinkholes than others -- and not all sinkholes are the work of nature.

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