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Grounds for concern

Published Sep. 10, 2005

The wisecracks began soon after the sinkholes.

Why not call the place Spring Hole? Maybe Sink Hill?

In the past two weeks, dozens of sinkholes have appeared in this western Hernando community, closing a major road, forcing the evacuation of homes and bringing closer inspections of backyard dimples and driveway cracks.

The rapid proliferation _ as many as 40 sinkholes in the area around Spring Hill _ are the result of a mix of conditions: drought, excessive groundwater pumping and torrential rains.

But those conditions are by no means unique to Hernando County. In fact, experts say, those conditions could repeat themselves just about anywhere in the Tampa Bay area.

"I don't know statistically how your chances compare to being struck by lightning. I wouldn't lose sleep over it," said Ann Tihansky, a hydrologist with the U.S. Geological Survey. "But there is definitely luck of the draw."

Spring Hill was a victim of what one expert called perfect "sinkhole weather" and a "recipe for disaster."

Specifically, Spring Hill was nailed on July 11 with 4{ inches of rain that swamped streets and sloshed water into drainage areas that drain densely populated neighborhoods.

Experts think the sheer weight of the water _ and the erosive force its sudden presence brought to the parched earth below _ led to a 60-foot-wide "mother" sinkhole along busy Mariner Boulevard.

The pressure caused by the main sinkhole probably opened dozens of smaller pockets elsewhere in the neighborhood, said Tony Gilboy, a sinkhole expert with the Southwest Florida Water Management District, also known as Swiftmud.

But it was the long drought that set the stage for a big collapse.

In simple terms: The buoyancy of water underground helps hold up the sand and clay subsurface _ a liquid hand pressing upward.

When the water is gone, so is a key part of nature's underpinning, not to mention nature's glue _ the moisture in the ground that holds loose soils together.

Residents here are well aware that the biggest hole opened in a man-made drainage basin and that one of the three condemned houses was damaged by a hole that opened when workers nearby began drilling for a well.

"When man starts altering things there's a tendency for things to start happening," said Gilboy.

The natural flow of surface water has been diverted and disrupted by everything from road construction to retention ponds, scientists say. And the water below ground that helps hold up the subsurface has been drawn down, in part, by pumping in Pinellas, Pasco and Hillsborough counties.

Of course, sinkholes were part of Florida long before people were.

When the first winter visitors came here 12,000 years ago, sinkholes were nibbling away at the ground beneath them. When Ponce de Leon strode ashore in the 16th century, his Fountain of Youth was probably being fed by water funneled from a sinkhole.

That's because Florida sits on a bed of limestone that has been pockmarked through the ages with holes, caverns and crevasses. Constantly, underground streams and spongelike pockets of water chew away at the rock, causing cracks, weak spots and, ultimately, sinkholes.

Experts say cave-ins are more likely when the clay and sand layers above the limestone are thin, as is the case in northern Pinellas and Hillsborough counties, western Pasco, western Hernando and much of Citrus County.

People can fill in the holes and use ground penetrating radar to scout out a solid piece of earth. But that won't change the fact Florida is perched on a honeycomb.

"You are building on land that probably Mother Nature doesn't want you to build on," said University of Florida geological sciences professor Anthony Randazzo. "And this is what we're experiencing."