DUNEDIN, Fla. - A boat listing astern on land usually brings bad news. The ground beneath it in Dunedin gave way, taking parts of two houses with it. Sinkholes happen in Florida, most of which sits on a bed of limestone, with a layer of soil, sand or clay on top of it. When it rains...
David Brown: "It percolates down through the soil column."
David Brown is a geologist. And he explains how water gets into the limestone, and over hundreds or thousand of years, slowly dissolves it away, creating cavities underground.
David Brown: "A house is placed at land surface, unsuspecting of some of the cavities that may lie underneath it at depth."
Water pressure helps prop up the land above it. When things dry out...
David Brown: "Water levels to drop, within this cavity, causing a collapse to occur."
In February, a sinkhole swallowed a man in Seffner. In August, a resort in central Fflorida partially sank into the ground. But we rarely see things like that here. A Florida geological survey map shows the number of reported sinkholes between 1954 and 2004. Dots cover Pinellas, Hillsborough and Polk Counties to our north. But only three appear in Manatee County, seven in Sarasota County, and one in Charlotte County.
David Brown: "Sarasota County our limestone is much deeper."
That's because, Brown says, the layer of clay covering it is hundreds of feet thick in some places – too thick for the rain to reach the limestone.
David Brown: "We're protected in this area by those confining units and the depth of our limestone."
Florence Conlan: "You definitely want to have the coverage for it."
Florence Conlan sells insurance, and says that rates for sinkhole coverage here do factor how few sinkholes we get.
Florence Conlan: "So they'll look within a mile-and-a-half. They'll look within three miles. And they'll see how many occurrances within that geographical area."
And they'll see what geologists see: little chance that the earth will open and swallow your home. David Brown says that most of the sinkholes we have here come from man – water pipes leaking – or something we buried settling and causing the land above it to sink down.