The Brule River splits in two just above a waterfall and falls in two cascades. That's not all that unusual; in fact it's pretty common. What's odd here is that one of the streams falls into a pothole. But wait! Even though that made me do a double-take, reading the text really blew my socks off. See, no one knows where the water goes, and a number of obvious tests have been done to solve the problem without useful results:
One [theory] is that, after dropping down the pothole, the river runs along a fault underground, or as a variant, that it enters an underground channel and comes out somewhere under Lake Superior. Both of these ideas have one valid aspect in common: they recognize that water must move downhill! But the main problem is creating a channel or conduit large enough to conduct the impressive flow of half the Brule River! Faulting commonly has the effect of crushing and fracturing the rock along the fault plane. This could certainly increase the permeability of the rock - its capacity to transmit water - but the connected open spaces needed to drain half the river would be essentially impossible, especially for such a distance. Furthermore, there is no geologic evidence for such a fault at the Devil's Kettle. Large, continuous openings generally do not occur in rocks, except for caves in limestone terranes. The nearest limestone is probably in southeastern Minnesota, so that doesn't help... Maybe the Devil's Kettle bottoms out fortuitously in a great lava tube that conducts the water to the Lake... Unfortunately for this idea, they are not the right kind of volcanic rocks! Rhyolites, such as the great flow at this locality, never form lava tubes, which only develop in fluid basaltic lava. Even the basalts in this area may not be the "right kind", being flood basalts that spread laterally as a sheet from fissures, not down the slopes of a volcano. No lava tubes have been found in the hundreds of basalt flows exposed along the North Shore. Furthermore, the nearest basalt is so far below the river bed, and even if it did contain an empty lava tube (very unlikely after its long history of deep burial) the tube would have to be both oriented in the right direction (south) and blocked above this site so that it isn't already full of debris. And there are no reports of trees or other floating debris suddenly appearing at one spot offshore in Lake Superior. The mystery persists.Wow. This is a delightfully puzzling planet.