Rock Bridge Memorial State Park

Rock Bridge Memorial State Park, on the south edge of Columbia, is karst country you can walk into. The rock bridge is a 125-foot natural tunnel left standing when part of a cave roof fell in, and the Devil's Icebox Boardwalk leads through it to the Devil's Icebox, a double sinkhole where cold air rises and the underground stream shows, then past Devil's Icebox Spring and back. Connor's Cave, downstream, can be explored on your own. Devil's Icebox Cave, upstream, is closed to protect its bats.

That cave has 6.5 miles of mapped passage. Endangered gray bats raise their young in it each summer, and it is the only known home of the pink planarian, a flatworm. Its 8,500-acre watershed holds thousands of sinkholes and losing streams that drain through the limestone into the cave, so the quality of water on the surface decides what lives below.

The stream that leaves the cave once powered a village. A stone dam at the bridge ran a gristmill, the first paper mill west of the Mississippi opened here in 1834, and a whiskey distillery followed in 1847, in a settlement called Rockbridge Mills and later Pierpont. The land stayed private but served as a public gathering place for plays, rallies and dances for more than a century. In 1961 Carol Stoerker, the nine-year-old daughter of a University of Missouri professor, was struck and killed by a car, and her father wanted a place where children could play in safety. The campaign that followed made it a state park in 1967.