It stays 57 degrees inside. While the Missouri summer bakes the surface, you can walk through dripping subterranean chambers filled with massive, active flowstones that look like melted wax. Water drips constantly here. Towering columns like the Queen's Canopy crowd the shadows, while a perfectly still pool of water reflects rare, round calcite deposits in the Lily Pad Room. It looks completely alien.

Once your fingers get numb from the cave air, you can head back up to launch a kayak into the cool, green waters of the Meramec River. The river runs right past. You can hike the steep, rocky bluffs on the Oak Ridge Trail to catch a sweeping view of the river valley. Pitch a tent after. The campground sits under thick oak trees, so you can fry up some bacon over an open fire and listen to the whip-poor-wills call out as the night takes over.

It started with a leaky mill. Back in 1886, a local miller named Charles Christopher noticed water draining into a sinkhole, which led him straight to a massive, hidden underground cavern. He did not do much. But in 1897, William Mendenhall bought the property, aiming to cash in on the upcoming 1904 World's Fair. Early tourists paid fifty cents. They floated in on rickety wooden flatboats, gaping at the dripping, active stalagmites under the dim, smoky glow of handheld lanterns.

Later, a legendary promoter named Lester Dill bought a share in 1932. He loved wild publicity stunts, even releasing helium balloons to find new passages, and he ran the site as a commercial attraction for decades. Then things got ugly. In the 1970s, the federal government planned a reservoir dam on the nearby river that would have drowned these delicate limestone formations forever. Locals fought the project. Thanks to their fierce activism, the state stepped in to buy the property in 1982, protecting both the subterranean system and the ruins of the 1880s Davis Mill. The water flows free now.