One way to enjoy nature on a hot, humid day is to visit a cave. Caves form in areas with thick layers of soluble bedrock (limestones and dolomites); while these rocks were deposited in ancient seas, hundreds of millions of years ago, most of the cave erosion occured during the wet climate of the Pleistocene, which began 2 million years ago. In areas with significant annual precipitation, the erosion continues today as rainwater and snowmelt drip through the fractured ceiling, creating stalactites, stalagmites and flowstones.
In the U.S., caves are most numerous in a swath across Kentucky, Tennessee and Missouri; Mississippian limestones are the dominant cave-bearing rocks in Kentucky and Tennessee while Ordovician dolomites predominate across the Ozarks of Missouri. Mammoth Cave, thought to be the most extensive cave system on the planet, stretches along the Green River Valley, in west-central Kentucky. Other interesting destinations include Carter Caves State Resort Park, in the Tygart Creek Valley of northeastern Kentucky, and Onondaga Cave State Park, in the Meramec River Valley of southeast Missouri.
Caves are also found in relatively dry areas of the Western U.S., having formed when the regional climate was much wetter. Wind Cave and Jewel Cave, both in the Black Hills region of western South Dakota, were carved from Mississippian limestone. Carlsbad Caverns, one of the most spectacular cave systems on Earth, developed within the Permian limestone of southern New Mexico.
All subterranean caves maintain a relatively even temperature throughout the year, offering a cool retreat in summer and a warm shelter in winter. Of course, wildlife discovered this fact long before man did and a wide variety of creatures use caves on a seasonal or permanent basis. Small caves (or the entry zone of larger ones) are favored by raccoons, ringtails, bears, bobcats, mountain lions and a variety of small mammals; snakes also inhabit these areas. Various bat species are the classic cave inhabitants, using some as hibernation sites and others (especially large, southern caves) as homes for huge, permanent colonies. Deep within the caverns, blind and depigmented creatures (fish, crawfish, crickets) have adapted to the perpetual darkness of these hidden ecosystems.
