'School Sinkhole' at Cave of the Mounds is named for the thousands of students who visit each year on field trips.
In August, a 60-foot-wide sinkhole formed under a resort in Florida, forcing the evacuation of about 35 guests before a three-story building collapsed and another sank. Last spring, a Manitowoc man had to be hoisted out of a sinkhole about three feet wide that wasn't there the day before.
"The idea of a sinkhole is frightening," says Dave Hart, hydrogeologist for the Wisconsin Geological and Natural History Survey. "You have the idea that your front yard is solid ground, and all of a sudden there is a hole in it. We are called about one or two sinkholes a month during the spring and summer."
Most are only a few feet and aren't reported. Hart estimates there are thousands of little sinkholes in the state and 20 to 30 big ones. "We advise people to try to fill them in. A soil engineer can check it to be sure it's stable. Start with larger rocks at the bottom, then smaller stone, then soil, but they often come back."
Here in Wisconsin, Hart is not concerned about vanishing houses, but he does worry about groundwater when he sees a sinkhole. The cracks and crevasses provide a direct conduit for pollutants. "We depend on soil to filter groundwater, and we lose that filter when a sinkhole forms. We need to be careful what we put on that land."
Wisconsin's sinkholes are relatively small because the bedrock beneath the state is harder than it is in Florida. However, both Florida and Wisconsin share karst landscapes -- bedrock, usually limestone or dolomite, prone to cracking.
In Wisconsin, this sedimentary rock was formed over 400 million years ago when the shells of tiny creatures fell to the bottom of an ancient sea and compacted into the soft stone. It can be seen in house foundations, walls and gravestones left by early settlers in this area.
But the bedrock beneath us can dissolve, becoming honeycombed with fractures that slowly widen. Sinkholes form from the bottom up, and they can open suddenly when sediment in deep cracks and voids washes away from below until the soil at the top is too thin to support its own weight.
Sinkholes exist in the Madison area and have become part of the educational curriculum for the 307-acre Madison School Forest, southwest of Verona. Madison teacher Jill Olsen and Nancy Sheehan, stream monitoring coordinator for the Rock River Coalition, created a series of lesson plans around the sinkholes. These are available to all area teachers for conducting hands-on science activities.
"More sinkholes have been discovered here since the first one, which is 20 feet across and 15 feet deep," says Sheehan. "Several are located along the walking trails, but they are not marked." Students collect data on the growth of the sinkholes, she adds.
"Sinkholes provide a window into the essential but invisible world below us of rock and groundwater."
Just to the west of Madison is an excellent place to view sinkholes. Much of Wisconsin's crumbly karst is cushioned by a thick layer of glacial deposits that makes sinkholes less likely, but the Driftless Area was not touched by the last three glaciers.
At Cave of the Mounds, in Blue Mounds, there is public access to five sinkholes, including one of the largest in the area. A stroll along Karst View Trail with an interpretive trail map leads to several of the sinkholes on the property, two of which can also be seen from below within the cave.
Brigham sinkhole, about 30 feet in diameter, is a fairly typical size for the area, but its depth can only be guessed since much of the collapse site has filled in and is now below ground. Ebenezer Brigham settled the area in the 1820s, establishing a smelting furnace that evolved into the family farm.
In the days before garbage pickup, farm families tossed generations of refuse like old crockery and glass into any handy ravine or, in this case, a handy sinkhole. Ann Wescott, co-manager of Cave of the Mounds, says that items tossed away years ago regularly resurface at the edges of this sinkhole. "We have found some very old bottles imprinted 'Brigham Farm Dairy,'" she says. "What they called garbage, we call artifacts."
The largest and most dramatic of the sinkholes, at least 40 feet in diameter, is still active. Named School Sinkhole because of the thousands of students on school field trips who have peered over its edges, this sinkhole is fenced because its bottom is unstable.
"Water is usually flowing beneath the ground," Wescott explains, "and when you have a really wet spring like we did this year, there is really a lot of water passing below the surface of the ground. This water dissolves the underlying bedrock, enlarging the cavity. Gravity then pulls the surface material down into that cavity, enlarging the sinkhole."
Just to the northwest is a small depression called Oscar's Sinkhole, named for the farmer who discovered it by accident over 30 years ago.
"Oscar was driving a tractor, probably cutting hay, and his front wheels fell right in," says Joe Klimczak, co-manager of Cave of the Mounds. "We would fill it in, except for its interpretive value in connecting the surface and subsurface of our environment."