In Illinois, caves can be found in four distinct karst regions:
Until now several hundred caves are known. There are no show caves in the whole state, but two of the caves may be visited on easy cave trekking tours.
During the Mississippian Period much of Illinois was covered by shallow seas. In those seas carbonatic sediments, limestone and dolomite were deposited.
In a few caves in Monroe and St. Clair counties, the Illinois cave amphipod lives, a small shrimp-like creature that exists nowhere else in the world. They inhabit the bottoms of pools and riffles in large cave streams, feeding on decaying leaf litter and organic debris. But the urbanization of the greater St. Louis metropolitan area, without appropriate sewage treatment and disposal, is threatening to the survival of those amphipods. Other serious threats are siltation and the presence of agricultural chemicals in subterranean aquifers.