The following species of bats that inhabit the caverns: Indiana bat, Gray bat, little brown bat. All together, these and more rare bat species such as Eastern Small-footed Bat had estimated populations of 9-12 million just in the Historic Section. A sandstone and shale cap, as thick as 50 feet in places, acts as an umbrella over limestone ridges. The umbrella leaks at places called sinkholes, form which surface water makes its way underground, eroding the limestone into a honeycomb of cavern. The caves natural temperature is 54 degreases, with a relative humidity of a high 87 percent. They contain underground lakes and rivers and numerous unique geological formations, including stalagmites and stalactites.