It's a nightmare scenario; you're at home—maybe even sound asleep in your bed—or perhaps just walking down the street, and all of a sudden, the ground beneath you literally opens up, swallowing homes, cars, trees and even people into a muddy, seemingly bottomless pit. If you're in California, you might attribute such a catastrophe to an earthquake, but in Florida, chances are you've just witnessed—or been the victim of—a sinkhole. Some regions of the country are prone to sinkholes because of their geologic makeup—and Florida is one such region. Believe it or not, sinkholes are more of an insurance risk in the Sunshine State than hurricanes. And Florida has more sinkholes than any other state in the nation.
The Hole Story
The first thing you need to know about sinkholes is that there are two different kinds. The one where the ground opens up and houses fall in, like in a horror movie? That’s called a cover collapse sinkhole, and they are—thankfully—relatively rare. “The ground and the soil on the top totally gives way and collapses down into the hole,” explains Jay Silver, president of Helicon Property Restoration in Tampa. “Over time, the soil is eroding, and there’s a void close to the surface.”
The other—and by far more common—type is a subsidence sinkhole. “That’s down at the limestone layer,” Silver says. “Over time, as the rain progresses down through the soil, it becomes slightly acidic, and it will erode away that soft, porous limestone layer and cause it to bow down and dip, and the upper soils will slowly migrate downwards. This will cause a structure at the surface to experience some light settlement and some damage.” The most famous example is the sinkhole beneath the Leaning Tower of Pisa, which modern Italian engineers have managed to arrest…but not arrest so much that the wonder of the world loses its trademark tilt. Subsidence sinkholes can be shored up—or at least made more safe.
Why is this horror movie scenario so likely in Florida? It has to do with the state’s geological makeup—and its rapidly increasing population. There have been more than 300 sinkhole occurrences reported in Florida since 2010, and in one tragic 2013 incident, a man was killed when a sinkhole swallowed up the bedroom underneath his Tampa home.
“Almost all of Florida is susceptible to sinkholes to some extent, because the entire state is underlain by several thousand feet of carbonate rock, and carbonate rock is the collective term for limestone and dolostone,” says Clint Kromhout, a geologist with the Florida Geological Survey. “These are both dissolved by weak acids, which are found in both our groundwater and rainwater. The difficultly is knowing which areas are more conducive to dissolution than others. What we’re trying to do is map the geologies out and provide that map to the Division of Emergency Management and to others.”