Three pigs found it. In 1924 they went missing from a farm outside Harmony, in the southeast corner of Minnesota, and the search turned up a sinkhole where the thin limestone under the soil had given way. Neighbour boys were lowered on a rope. Seventy five feet down they found the pigs alive, and behind the pigs a labyrinth of underground canyons nobody knew was there.
Tours started in 1934 and still set off from the same spot the pigs landed in, now called the Reception Room. The route runs about a mile underground and drops to nearly two hundred feet below the fields, which is deep enough that the cave keeps its own weather: it sits at a constant forty eight degrees Fahrenheit whatever the season is doing above it. Take a layer even in August.
The centrepiece is a sixty foot waterfall running inside the rock, fed by a stream that is still cutting the passage it falls through. The cave is worked into Ordovician limestone of the Galena formation, and the fossils in the walls are roughly 450 million years old, which is to say the seabed they came from predates anything with a backbone walking on land.
It is a commercial cave and it leans into it. There is a subterranean wedding chapel down there, and people do use it.
Guided tours run about an hour. The season is 1 May to 1 November, seven days a week, with some days added in November, so this is a spring to autumn trip rather than a winter one. Check the operator before driving out: the dates shift year to year and the cave is a family business rather than a park with fixed hours.
Other places on the map within a couple of hours.
