[SciShow intro plays]

Michael: You’d think, when water tumbles down a waterfall, that you’d be able to see where it’s going. Even questioning it seems absurd. Like, the answer is right there in front of you!

But Devil’s Kettle Falls in northeast Minnesota isn’t like most waterfalls -- it seems to just disappear into the ground. The water in the falls comes from the Brule [brool] River, which splits off as it meanders down toward Lake Superior. Water flowing down the east side of the Brule cascades down a perfectly normal waterfall and continues on to the final stretch of its journey to the Great Lake.

The west side, though... that’s a different story. After running along an outcrop for a few meters, the water tumbles into a rocky pothole -- also known as a kettle. And -- as far as anyone can tell – it’s never seen again!

The water that keeps gushing into the kettle must be going somewhere, or it would overflow like a bath left running with the plug still in. But nobody knows where that might be. There are all kinds of stories about people throwing in ping-pong balls or putting colored dyes into the falls, which never showed up again. And there haven’t been any official experiments, so for now, all we have are educated guesses.

But for every sensible-seeming idea that people come up with, there are reasons why they can’t apply to Devil’s Kettle Falls. One possibility is that the West half of the Brule River passes through an underground cave in the rock, running through channels in the rock until it meets Lake Superior. Strong river flows often form underground caves, especially in limestone. Limestone is made from calcium carbona--te, a chalky compound that’s slightly soluble in water. So, over time, the action of gushing water can erode the rock into some spectacular caverns.

But there is a pretty big problem with this theory, though: northern Minnesota doesn’t have the soft limestone needed to make these caves. Instead, the area is mainly volcanic igneous [ig-nee-iss] rock, the solidified flow of ancient lava. This rock tends to be much harder than limestone and won’t form large internal caverns, but some kinds of igneous rock do form lava tubes. And it’s possible that the water from Devil’s Kettle Falls is using some of those lava tubes to get around, and eventually join up with Lake Superior.

Lava tubes form when the surface of a lava flow cools in the surrounding air and solidifies. Underneath, hot lava keeps flowing, insulated by the crusty layer on top. When the eruption ends and the liquid lava passes through, there’s a hollow tunnel left over If you visit the KÄ«lauea [KILL-uh-WAY-uh] volcano in Hawaii or the Lava Beds National Monument in California, you can see -- or even go inside -- some of the more impressive lava tubes.

And the tubes are common in places like Oregon, where they’re known as the cause of the so-called “Disappearing Lake” that drains away every summer. But you won’t find any lava tubes around the northern shores of Lake Superior. Again, the problem is that it’s the wrong type of rock for that. Lava tubes generally form in basalt lava, the fastest-flowing kind. It has less silicon dioxide than other types of lava, which makes it runnier. But the rock around Devil’s Kettle Falls is rhyolite [RYE-o-lite], a pale pink or grey rock that’s much higher in silicon dioxide. So it comes from a type of lava that flows much more slowly.

Because rhyolite is so lethargic when it flows, it doesn’t form hollow tubes – you need speed for that. So neither of the running theories for where the water goes really makes sense. And it’s hard to tell if the falls connect up with Lake Superior at all. The Brule River isn’t exactly small, but the lake reaches 406 meters deep and has a surface area roughly the size of Austria. In such a huge lake, the extra water from one river isn’t so noticeable. So we probably won’t learn the fate of Devil’s Kettle Falls until some researchers get down there and try to track the water that’s running with the Devil.

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