How do sinkholes form?
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.
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.
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:
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.
Updated March 21, 2012