Devil's Kettle on the Brule River is a sinkhole that swallows half the river and the water just disappears. For the longest time it was a huge mystery where the water went, as there was no obvious outlet. Speculation ranged from cave networks to underground aquifers we're abundant, until in 2017 it was proven that the water flow above the falls was equal to the flow going into Lake Superior.

Now the only question is where the water rejoins the rest of the river.