Florida tragedy: So what makes a sinkhole open up?

Imagine a limestone gorge. Look back in time and imagine it covered over. And then think what happens at some time between the two. Imagine standing above ground at that stage and discovering the first hole in what was then a cavern 'roof' when the ground opened up. And you're thinking of a sinkhole.

Limestone is slightly soluble, allowing water to disappear underground where it erodes away rocks to form huge caverns - and those open into huge holes or gorges as the roof gives way.

Sinkholes - sometimes known as snake holes and swallow holes - also occur in sandstone and where underground water moves away soil. They occasionally have man-made causes, such as mining. The holes might be a metre wide and deep - or hundreds of metres across and down.

Anyone who has been to Malham Cove in North Yorkshire can imagine the process clearly as they look up the gorge to where the stream emerges from the base of the limestone cliff, standing where once was a huge cavern.

Although these voids may form very gradually they often appear suddenly when the final crust gives way - and that is what seems to have happened in Seffner, Florida, where a 6-metre hole opened up beneath the home of Jeff Bush.

“There's hardly a place in Florida that's immune to sinkholes,” said Sandy Nettles, who owns a geology consulting company in the Tampa area. “There's no way of ever predicting where a sinkhole is going to occur.”

Most sinkholes are small, like one found Saturday morning in Largo, 35 miles (56 kilometers) away from Seffner. The Largo sinkhole, about 10 feet (3 meters) long and several feet wide, is in a mall parking lot.

The state sits on limestone with a layer of clay on top. The clay is thicker in some locations - including the area where Jeff Bush became a victim.

Jonathan Arthur, the state geologist and director of the Florida Geological Survey, said other states sit atop limestone in a similar way, but Florida has additional factors like extreme weather, development, aquifer pumping and construction. “The conditions under which a sinkhole will form can be very rapid, or they can form slowly over time,” he said.

What specifically caused the Seffner sinkhole? No-one can be sure.

“The condition that caused that sinkhole could have started a million years ago,” Nettles said.

Staff and AP