The Judge C.R. Magney State Park on Minnesota’s north shore of Lake Superior, just a few miles south of the border with Canada, has several waterfalls. One of them has mystified geologists and hikers for decades, Caitlin Schneider reports for Mental Floss — because once the water falls, it simply disappears.

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The Brule River flows through the park, dropping 800 feet in eight miles as it carves its way toward the lake. At Devil’s Kettle Falls, a "thick knuckle of rhyolite" (a volcanic rock) splits the river in two, reports Stacie Boschma for MNN.com. One side streams over the rocks like a normal waterfall, but no one knows where the second half of the river, which drops into a deep hole, ends up.

Schneider writes that though scientists think the river must drain somewhere beneath Lake Superior, nobody knows for sure. Over the years, she writes, "researchers and the curious have poured dye, pingpong balls, even logs into the kettle, then watched the lake for any sign of them. So far, none has ever been found."

An intrepid Youtube-r, Travis Boser, took a video from a perch between the diverging river that peeks into the Devil’s Kettle. A few logs can be seen at the bottom of the hole. In the comments, Boser notes that the river was low at the time of his recording, a flow that allowed him to scramble up to that viewpoint.

People have proposed a few possible explanations, but the trouble is that the geology of the area doesn’t support them. Caves and underground channels most commonly form in limestone rocks, which dissolve easily in water. But the park rests on layers of basalt and rhyolite, which erupted when the North American continent started to rift apart 1.1 billion years ago. The rift failed, but left behind a huge curving basin that now holds Lake Superior.

Some think the hole is the opening of a lava tube — but rhyolites never form lava tubes. Basaltic lava does, but those rock layers are far below the river bed and of the wrong type — they are sheets of flood basalt, not the kind of flows that typically form tubes. Even if one did somehow form, it would be strange for it to extend all the way to the lake and never grow clogged with sediment, trees and other debris. An open fault line, another offered explanation, would face the same problem.

The disappearing river remains a mystery. Whether or not you think you can solve it, you can take a look for yourself: viewing it in person requires a mile-long hike and a climb of 200 steps, but should be worth the effort.