This is not a karst cave. Cave-in-Rock is a tunnel 55 feet wide worn straight into the limestone bluff of the Ohio River by floodwater, much of it glacial meltwater after the Wisconsin ice age, and shaken further by the New Madrid earthquakes of 1811 and 1812. Unlike Mammoth Cave over in Kentucky, no slow dissolving of rock produced it. The river simply cut a hole in the cliff, at the water's edge, facing the channel.
That geography is the whole story. Native people knew and used it for thousands of years. The first European to record it was M. de Lery of France, who mapped it in 1729 and called it caverne dans Le Roc, which is the name it still carries in translation. It has also gone by Rock-In-Cave, House of Nature, Potato Cave and Murrell's Cave, among others.
Its notoriety came in the 1790s and the first thirty years of the nineteenth century. Flatboats loaded with produce from Kentucky, Ohio and southern Indiana floated down the Ohio toward the market at New Orleans, and a cave mouth opening directly onto the river, upstream of a bend, made an ideal place to watch for them. River pirates, counterfeiters and outright murderers worked out of it, most notoriously Samuel Mason, who set it up as a tavern to draw crews ashore. Boats were taken, crews were robbed and cargoes were sold on downriver.
The cave is now the centrepiece of a 204 acre Illinois state park in the town of Cave-in-Rock, Hardin County, run by the Illinois Department of Natural Resources, with a bluff-top campground and a working ferry across to Kentucky. The cave itself is free, open and a short walk from the car park.
Other places on the map within a couple of hours.
