MADISON — Sinkholes have been much in the news lately and, probably, much in the nightmares of those who are particularly susceptible to the fear of being suddenly consumed by a yawning, bottomless hole opening up in the Earth beneath their feet.
Even Dave Hart gets a little shaky thinking about it. And he’s a hydrogeologist with the Wisconsin Geological and Natural History Survey.
With recent stories of a man dying in a Florida sinkhole and, just this week, an Illinois man surviving being swallowed on a golf course, Hart said it is only natural for people to think the worst. “This hole opens up in the ground, and you’re going in all the way,” he said of how some react.
Now for some more nightmare fodder. Sinkholes do happen in Wisconsin. And they most commonly happen in the spring, especially when a heavy snowcover is melting.
The good news, according to Hart, is that Wisconsin sinkholes, because of the underlying geology, are generally not the giant, gaping man-eaters that are sometimes seen in Florida and elsewhere. No records exist of sinkholes having killed anyone in Wisconsin as far as Hart knows.
Nevertheless, Hart said, Wisconsin has a large V-shaped area from Green Bay down to Madison and Dane County and back up to St. Croix Falls that is underlaid by a kind of rock called dolomite and susceptible to sinkholes. It works like this. The dolomite is easily dissolved by water so that it becomes permeated by holes. Water then washes the soil above the rock down through the holes and cracks creating the sinkhole collapse on the surface. It’s the same process, Hart said, that has created the caves, such as Cave of the Mounds, that are a feature of southwestern Wisconsin.
Wisconsin sinkholes are not as large as those in Florida and other places where the bedrock is limestone, which is even more soluble than dolomite. As a result, Wisconsin does not have the “huge, vertical conduits” that can open up in the limestone, Hard said. Also, he added, Wisconsin’s bedrock is often interlaced with other kinds of harder rock that keep sinkholes from becoming too large.
Hart recalled inspecting a sinkhole a few years ago in rural Dane County that was about five feet wide and 20 feet deep. But it seemed to be unstable as he was measuring it and he remembers being nervous and stepping gingerly. It had almost gulped down a farmer who was busily spreading manure when he felt a strange and ominous tug.
“The farmer was pulling a manure spreader and he felt a wheel drop and he floored it and got out of there just in time,” Hart said.
Spring, especially, is sinkhole time because of the melting snow and the larger volume of water draining off the landscape, Hart said.
Hart, being a geologist, is fascinated by the phenomenon. And he believes most, at heart, are also intrigued.
“Underneath us,” he said, “there is this whole other world. It reminds us of that.”