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Earth in Action

Geologists develop weapons to combat that sinkhole feeling

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What do five Porsches, several Kentucky thoroughbreds and a three-story building in Guatemala City have in common? They’ve all been swallowed by sinkholes.

Sadly, the sudden cave-ins sometimes claim people’s lives as well. On February 28 the earth opened up underneath the Seffner, Fla., bedroom of Jeff Bush, entombing him. The freak accident highlighted Florida’s vulnerability to sinkholes, and the seemingly sheer randomness of death by earth.

But geologists are fighting back. The battle isn’t just one man versus the ground; it’s science versus society’s tendency to put structures in harm’s way.

Sinkholes are just one manifestation of a much larger geographic phenomenon known as karst. You’ve seen karst landscapes if you’ve been through the Hill Country of central Texas or to Mammoth Cave in Kentucky. Karst can form anywhere you get rock that is easily dissolved —  like limestone or its chemical relative, dolomite — and water draining through that rock.

Runoff from rain, streams or lakes percolates through the soil and picks up carbon dioxide on the way, becoming slightly acidic. The acid reacts with the soft rock and chews away at it, widening tiny cracks into larger fissures. Eventually, the subterranean landscape can get honeycombed with caves, chambers and other hollows. If your house is right atop one of those buried empty spaces, you may be in trouble — because the fragile barrier between yourself and the void can easily give way.

Karst is common stuff, making up some 20 to 25 percent of all the land surface on Earth. Roughly 40 percent of the United States east of Oklahoma is karst, including large swaths of Pennsylvania, Tennessee, Kentucky and Georgia.

And, of course, Florida. Nearly the entire state sits on a thin veneer of limestone and dolomite rock. Water, too, is key; drain underground aquifers for drinking or agriculture, and the ground suddenly becomes more unstable and prone to collapse. During a cold snap in 2010, farmers in the state strawberry capital of Plant City pumped millions of gallons of underground water onto their crops to save them — but ended up causing dozens of sinkholes. Some popped up perilously near the interstate, and one Plant City woman nearly got sucked into her backyard twice, both that year and the year after.

The litany of sinkhole disasters in the Southeast reads like a horror novel for insurance executives. Those thoroughbred horses? They vanished among the bluegrass country of Kentucky. The five Porsches? They met their end in Winter Park, Fla., a manicured suburb near the family playgrounds of Orlando, when a 100-meter-wide hole opened suddenly on May 8, 1981.

State legislators created the Florida Sinkhole Research Institute the following year. But it lasted for only about a decade before people once again forgot about the threat beneath their feet. Now, the Florida Geological Survey maintains the only database of sinkholes across the state — or what it calls “subsidence incidents,” as most have not been checked by a professional engineer.

People are going to keep moving to karst-rich regions, and keep on draining the water out of them. The question is whether scientists can do anything about the sinkholes that are sure to follow.

There are some glimmers of hope. Engineers in Italy and Spain, two countries with some spectacular landscapes underlain by karst, have developed new methods to predict which areas are most likely to fail first. Italian scientists recently combined ground-penetrating radar and electrical studies of the soil to spot buried anomalies that may represent earth about to give way. In northeastern Spain, researchers used mapping software to combine dozens of layers of geographic information and pinpoint which areas are most susceptible.

In New Mexico, students of sinkholes are even looking to space. After a salt well collapsed in the town of Artesia in 2008, environmental engineers started probing whether similar wells in other towns may also be at risk. The researchers used radar signals bounced off the ground by satellites that measure how long the pulses take to return to space. This technique can determine whether a spot on the planet’s surface is rising or falling over time, such as near a volcano on the verge of erupting or a sinkhole about to form.

Luckily, the satellite data showed that all is well in New Mexico — at least for now, the researchers report in an upcoming issue of a journal called, yes, Carbonates and Evaporites. But Florida can’t say the same. In late March, a second sinkhole opened in Seffner. It is just two miles from where the ground killed Jeff Bush in his bed.

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