Science of sinkholes: why do they happen, and why right now?

5:37 PM, Jan 14, 2010   |    comments
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Lakes created by sinkholes dot the landscape in Central Florida in this image from Google Maps.

St. Petersburg, Florida -- At least 30 sinkholes have opened up around Tampa Bay this week alone, costing taxpayers about $100,000 apiece. What's causing all these craters?  

Peer down from space at the Sunshine State, and you'll see -- especially toward the middle -- that we're also the Sinkhole State.

Photo Gallery: Sinkholes in Plant City, Lake Wales & Frostproof

Hundreds of nearly perfectly-round lakes are the results of sinkholes, just like the ones that are currently swallowing houses and highways in Plant City and elsewhere.

So, why does that happen here?

You probably think that under your feet in Florida, there's a layer of sand and soil, sitting on top of some sort of hard rock -- think of a slice of bread sitting on a tasty block of solid Monterey Jack cheese.

Well, in some places, that hard rock is more like holey Swiss cheese. Thousands of years of rain have carved cracks and caves in the bedrock.

You may think those big, empty caves would collapse. They don't because normally, the caves are filled with water -- and that water pressure is powerful enough to keep the ground above it in place.

But over the past two weeks, scientists say that groundwater, has vanished. In some cases, it has happened at record rates.

Charts showing the water levels in wells in Polk and Hillsborough counties have blue lines that hold constant for weeks, then plummet as they reach the first days of January.

Ann Tihansky, with the U.S. Geological Survey in St. Petersburg, says that major change is man-made.

"The lowered groundwater levels are due to pumping to protect crops from freezing temperatures," she said.

Farmers use well water to give their plants a protective igloo coating, or just to keep them warm, since well water is a constant 72 degrees.

Tihansky says research shows when that water's quickly pumped out of wells, the slow, natural process of sinkhole formation can be sped up dramatically.

"We're triggering it by decreasing groundwater levels over short periods of time," she said.

If even a small one of those Swiss cheese cracks is empty and close to the surface, the loose sand above rushes down in to fill it up -- moving like sands through an hourglass -- to create a sinkhole.

If a sinkhole appears under your home or a road, engineers can try several solutions to stabilize the ground.

They may bring in dump trucks full of dirt to fill in the hole, which often does the trick. In more challenging cases, engineers may call in a cement mixer to fill the hole with concrete or even inject some of the stuff into the soil under a house or street.

On top of those methods, engineers may sink tall pilings into the soft ground below the house. The pilings reach all the way down through the sand and into the hard bedrock below, to anchor the house to a more permanent material.

Photo Gallery: Sinkholes in Plant City, Lake Wales & Frostproof

Use this database below to check for sinkholes in any Florida county - just enter your county:

More Sinkhole Coverage:

Connect with 10 Connects multi-media journalist Grayson Kamm on Twitter as @graysonkamm, on his Facebook page, by e-mail at this link, or on AOL Instant Messenger as screen name GraysonConnects.

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