Mammoth Cave is the longest cave on Earth, more than 685 kilometres of mapped limestone passage under the hills of central Kentucky, in the United States, still growing every year as cavers push further, with eyeless fish and crayfish in its underground rivers. The cave is stacked in levels, dozens of horizontal tiers of passage one above another, each drained in turn by the river below as it cut deeper, so that the tours walk through dry upper corridors while the Green River still works far beneath; the passages are wide, flat-roofed and dry, more like tunnels than grottoes, and the scale is the first thing: halls the size of railway stations with the lamplight lost in them. People have been coming since 1816. The Historic Tour follows the route that has carried visitors for two centuries, past the wooden pipes and leaching vats left in 1812 by the enslaved men who mined saltpetre here for gunpowder, and past the names smoked onto the ceiling by candle.
Much of what visitors see was first explored by Stephen Bishop, an enslaved man who guided here from the late 1830s, crossed the Bottomless Pit on a ladder, found the river and its blind fish, and came to know the cave better than anyone alive; his map doubled the known cave, and his grave is at the entrance. At Frozen Niagara a curtain of flowstone built drop by drop over thousands of years towers like a waterfall frozen solid, and in the dry upper passages of Cleaveland Avenue delicate white gypsum crystals curl from the walls and ceiling like petals of frost. At one point on every tour the guide turns off every light at once, and the darkness is total, a black that the eyes never adjust to, with only the drip of water. Above ground the park is forest and the river, with more than a hundred kilometres of trails. Mammoth Cave is a country under a state, and nobody has found the end of it.