Sinkholes in Wisconsin

Click on a photograph to enlarge it.
A tractor-sized sinkhole near Eagle, Wisconsin. It appeared the day after the field was harvested.	A sinkhole, now patched, that formed in a highway near Norwalk, Wisconsin.	This quarry and opening are at the same elevation as the highway at left. They illustrate the fractured nature of the rock (karst) under the road.
A large sinkhole near Blue Mounds, Wisconsin.	This sinkhole is extremely dangerous. At 5 feet wide and 20 feet deep, it would be impossible to crawl out without assistance.	Before and after photos of a sinkhole-ridden retention pond in Madison. The pond drained out this sinkhole within a day (sinkhole photo courtesy Joe Demorett, Madison Water Utility).
Visit our photo gallery to view photos of other types of karst features in Wisconsin.

LISTEN (RealAudio file) as Dave Hart of WGNHS and Joe Klimczak of Cave of the Mounds discuss sinkholes and caves with Larry Meiller.

How do sinkholes form?
Sinkholes are holes or depressions that form when water washes sediment down into cracks and voids in karst bedrock. Karst is a landscape created when water dissolves rocks. In Wisconsin, dolomite and some limestone are typical soluble rocks. The rocks are dissolved mostly along fractures and create caves and other conduits that act as underground streams. Water moves readily through these openings, carrying sediment (and pollutants) directly into our groundwater.

Sinkholes form from the bottom up as the sediment immediately above the bedrock is the first to be washed into the voids. The land above a sinkhole often appears normal until a critical amount below has been washed away. When the soil surface can no longer support the weight, it collapses.

Not all sinkholes are the result of karst. Manmade sinkholes occur when a water main break washes sediment out of the area, creating a large cavity.  

Where do sinkholes occur?

Sinkholes occur worldwide, with some of the largest in Florida and Guatemala.

In Wisconsin, sinkholes are most likely to occur in a V-shaped swath that extends southeast from St. Croix County along the Mississippi River, across the bottom two tiers of counties, and northeast along Lake Michigan up to Marinette County. (Click on the map to enlarge it.)

Read more about karst and shallow carbonate bedrock in Wisconsin.

For details about where karst occurs worldwide, visit http://en.wikipedia.org/wiki/Karst_topography.

How big do sinkholes get?

Depending on the type of underlying bedrock, sinkholes can range in size from tiny depressions in the surface to gaping building-eaters that are hundreds of feet wide. Sinkholes in Wisconsin tend to be smaller than 10 feet across. The depth of sinkholes can be highly variable, although most are about as deep as they are wide.

Do I need to worry about my house falling into a sinkhole? The short answer is that it’s highly unlikely. Although other parts of the world have house-eating sinkholes, Wisconsin’s sinkholes are relatively small. The difference lies in our geology.  In Wisconsin, the karst bedrock forms in dolomite, which is much less easily dissolved than the limestone that forms the karst bedrock in Florida. As a result, we have fewer and smaller voids and cavities in Wisconsin's karst. The sinkholes are proportionally smaller as well.

Sinkholes and groundwater contamination

The cracks and crevasses in karst act as direct conduits for pollutants to enter groundwater, wells, springs, and streams. If you’ve got a sinkhole, you’ve got karst.

Protect your groundwater and wells by being careful about what you spread in these areas.

What to do if you’ve got a sinkhole

If a sinkhole appears on your land, first determine whether it’s a safety hazard. If it is, mark the location, restrict access, and, if necessary, call 911. Don’t allow anyone to crawl into the hole or be lowered into it—newly collapsed sinkholes may still be unstable. Also, please do not toss trash or anything you wouldn't want in your drinking water into a sinkhole.

Your next step depends on the size of the hole:

• Small sinkholes (less than 20 feet across)—The best strategy may be to fill it using cement or a procedure known as reverse grading where you fill the hole first with large rocks, then using progressively smaller rocks before topping it with soil. Randomly filling a sinkhole with soil and rocks will not permanently fill it.
• Large sinkholes (more than 20 feet across)—It may not be economical to fill large sinkholes. Instead, fence the area off to permanently limit access and ensure that no homes or other buildings are ever built on it. To prevent unfiltered surface runoff from entering the groundwater system, build up a low earthen berm around the hole.

A final note: Don’t be surprised if more sinkholes form nearby or if the same sinkhole returns. No matter how thorough the efforts are to fill a sinkhole, the conditions that created it—water and karst bedrock—are still present.

Help us catalog the locations and size of sinkholes and other karst features in Wisconsin. Please take a few minutes to submit a report and upload photos of the feature using our online karst reporting form.

More about karst
Report a karst feature
Links to more information about sinkholes and karst

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Updated March 21, 2012
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