Why so many sinkholes in Tampa Bay area?
TAMPA - The Tampa region sits atop one of Florida's most active sinkhole zones. The risk your house will be damaged by a sinkhole depends on where you live and, more importantly, what's under the ground there. Along the Hillsborough River valley — from the Green Swamp to the Tampa peninsula — sinkholes are shallow, slow-motion affairs caused by sand filtering into the porous limestone beneath. In eastern Hillsborough County, an ancient clay layer separates the sandy surface from the state's cavern-filled limestone foundations.Where that clay is thick, 200 feet or more, sinkholes are exceedingly rare. That's the case from the Alafia River south to Sarasota County. Where the clay is thin, however, the ground has the potential to give way suddenly, creating what geologists call a cover-collapse sinkhole. That's what swallowed Seffner resident Jeff Bush and part of his family home Thursday night. Cover-collapse sinkholes are impossible to predict, said Mark Stewart, a geologist at the University of South Florida. It's impossible to say what might have caused Thursday's sinkhole, Stewart said. "Fortunately, a big cover-collapse sinkhole is a rare event," he said. That's in geologic, big-picture terms. In the small picture — the past couple decades, for example — sinkholes have been anything but rare in eastern Hillsborough County. The Department of Environmental Protection has recorded more than 70 sinkholes within 5 miles of the Bush house since 1982. One of those swallowed most of a duplex a couple of miles from the Bushes' house in 2000. "It broke in two," said Richard Howe, whose late father, Willis, owned the property at 5325 Lemon Ave. By the time the hole stopped growing, it was 30 feet wide and 30 feet deep. A tenant who rented half of the duplex escaped unharmed, Howe said. But the building had to be demolished and the hole filled with tons upon tons of earth. Howe, who lives in central Michigan, and his siblings are trying to sell the empty lot.
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