Mammoth Cave is the longest cave on Earth, and the reason is that it was never one hollow. More than 685 kilometres of passage have been mapped under the hills of central Kentucky, nearly twice the next longest. The cave is stacked in tiers. As the Green River cut its valley deeper, the water table dropped and the cave cut a new near-level floor at each stage, leaving the older passages dry above. A hard sandstone cap keeps them from collapsing, so dozens of those storeys sit one above another, and you walk the dry upper ones while the river works below. The passages are wide and flat-roofed, more like tunnels than grottoes, and the air holds twelve degrees in every month.
The Historic Tour follows the route visitors have taken since 1816. It passes the wooden pipes and vats left in 1812 by the enslaved men who mined saltpetre out of the floor, then opens into Broadway, a canyon passage where the ceiling climbs past your lamp and the floor runs level for hundreds of metres. A curtain of flowstone called Frozen Niagara hangs amber in the lights, built drop by drop. In the dry corridors of Cleaveland Avenue, gypsum crystals curl out of the walls like frost, pushed out grain by grain where almost no moisture reaches. In the black streams below live shrimp and fish that have lost their eyes and their colour. At one point the guide turns every lamp off at once, and the dark is complete enough that your eyes never adjust to it and you lose the size of the room. Most of these routes were first found by Stephen Bishop, an enslaved guide who came here as a teenager in the late 1830s. He crossed a pit that had stopped everyone before him, reached the underground river and roughly doubled the known cave, then drew the map from memory. Every survey since has been added to the map he started.