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Sinkholes provide a lesson in geology

Now coming to a theater near you, Sinkhole: The Terror Below.

This cliffhanger may never find a place alongside such natural-disaster flicks as Twister and Earthquake. But if the movie ever is made, it very well could be set somewhere around here.

The massive sinkholes that recently emerged in Spring Hill and at Heritage Pines just across the Hernando border in Pasco County reminded area residents that the North Suncoast is prime sinkhole territory.

Some experts called the sinkholes, which were among the largest ever recorded in the area, an aberration.

But Anthony Randazzo, a professor of geology at the University of Florida, said a variety of conditions, such as the area's geology and burgeoning residential population, make it ripe for sinkholes. And, he said, the sinkhole problem is going to get worse.

Triggers of the recent sinkholes have been blamed on factors such as El Nino's torrential rains followed by a recent dry period, new development, overpumping of wells and drilling. But tracing the root of the region's propensity for sinkholes requires a look back into history _ way back. Back when a home in Brooksville would have been beachfront property.

Some 10-million years ago, clay from the Appalachian Mountains made its way all the way to Florida and spread across the region. The clay settled above a layer of limestone with pockmarks like Swiss cheese.

Think of the clay layer as a roof, said Tony Gilboy, a geologist with the Southwest Florida Water Management District. The clay roof can easily bridge a small hole in the limestone beneath it. But as the hole grows, the roof becomes less stable.

Water pressure is another factor. Say torrential rains, such as El Nino brought this winter, fill a cavern with water. When that is followed by a dry period _ like now _ the water level drops and releases pressure sustaining the clay roof.

There's another factor, a man-made one. The weight of a house or other structure can tax the clay roof.

But back to the history lesson.

Over the years, the ebb and flow of seas wore down the layer of clay in this region, particularly along coastal areas we now know as the North Suncoast.

Brooksville and much of the eastern part of Hernando County are largely immune from sinkholes due to a thick layer of clay along a geological formation called the Brooksville Ridge.

Moving west through Spring Hill, the clay layer pinches out and thins, Gilboy said.

The thinner the layer of clay, the weaker the roof and the more likely it becomes a sinkhole will occur. But while they may occur more often, they also tend to be smaller, perhaps a foot to several feet in diameter. Hence the garden variety sinkholes that often occur in the western parts of Spring Hill, Gilboy said.

As the clay layer gets thicker, a sinkhole is less likely to occur. But when it does, it is likely to be bigger. That explains why the sinkholes that occurred on Linden Drive and in Heritage Pines were so large.

"It's a function of what was there and what covered it over and what's there now," said David Rhodes of the state Department of Environmental Protection.

The state's most famous sinkhole, the Great Winter Park Sinkhole, occurred in 1981 in an area where the clay layer was thick. The 350-foot sinkhole caused about $4-million in property damage.

The national spotlight on the sinkhole sparked the formation of the Sinkhole Research Institute, based at the University of Central Florida in Orlando. The institute tracked reported sinkholes and tried to educate the public about them.

But as time went by and no more sinkholes opened up, at least none anywhere near the size of the Winter Park sinkhole, people lost interest. The institute lost its funding. It is now defunct.

Had the recent Spring Hill sinkhole occurred in Orlando or Tampa, Randazzo said, the institute would be revived.

Instead, tracking area sinkholes has fallen to Swiftmud. The agency offers technical assistance, but it doesn't plug holes. That's a property owner's responsibility.

Most of the calls to Swiftmud about sinkholes come from residents of Spring Hill, Port Richey and New Port Richey. Most are small, perhaps a few feet wide, and require only a small amount of fill.

_ Times researcher John Martin contributed to this report.