Historic Nickajack Cave, partially flooded by its namesake lake, is considered to be biologically one of the most important caves in Tennessee. This is primarily because of the thousands of gray bats that inhabit it from late April through early October, with peak numbers being in June and July. The gray bat, a medium-sized short-eared species, is one of over a dozen in Tennessee and is listed as federally endangered.
During the time of year they roost in Nickajack Cave, which is also when they give birth to their young, they emerge each evening after sundown from the mouth of the cave to search for food in a continuous stream estimated to contain over 100,000 bats and lasting about 45 minutes. It is estimated they consume 274,000 pounds of insects a year. Although they prefer mayflies and stoneflies, they also eat mosquitoes, caddis flies, beetles, moths and other aquatic insects. In the fall, the bats move to cooler caves for hibernation.
The cave has been closed to prevent diseases affecting the bats and now the best way to experience them is a guided kayak tour.