OK, slight exaggeration – Wikipedia says: ‘Judge C. R. Magney State Park is a state park of Minnesota, USA, on the North Shore of Lake Superior…best known for the Devil’s Kettle, an unusual waterfall and rock formation in which half of the Brule River disappears into a pothole.’
So where does it go?
The river splits in two to flow around a mass of rhyolite rock. The eastern flow goes over a two-step, 50-foot (15 m) waterfall and continues downstream. The western flow surges into a pothole, falling at least 10 feet (3.0 m), and disappears underground.
It is believed the water rejoins the main channel of the river or has a separate outlet into Lake Superior, but it has never been located. Researchers have dropped brightly colored dyes, ping pong balls, and other objects into the Devil’s Kettle without result.