A 2011 study found more than 13,000 visible sinkholes in the Illinois Sinkhole Plain, which primarily lies in western Monroe County. Some areas there have a sinkhole density of 95 per square kilometer.
The Illinois sinkhole plain developed between two million and 12,000 years ago, during the ice age, geologist Philip Moss said. The local topology is rare because the bedrock became covered with wind blown silt -- known as loess -- from the Mississippi River. The river was very wide and shallow during the ice age. The low water levels in the winter exposed the silt to winds from the west, which carried the soil into the sinkhole plain. Sinkholes have developed there since that time.
Unlike the rapidly developing sinkholes in Florida, in which there is a bedrock collapse, those in Illinois formed over thousands of years as holes formed in the soil and migrated toward the surface. Sinkholes open up several times a year in farm fields in the area, but they usually open during a storm, which is why people aren't typically at risk of getting trapped, Moss said.
Although they form over thousands of years, Illinois sinkholes, like the one that swallowed a golfer on the 14th hole of the Annbriar Golf Course in Waterloo on March 8, open rapidly. That hole has since been filled and the hole is open.