Judge C. R. Magney State Park is in Minnesota, east of Duluth, sitting just off the north shore of Lake Superior. (Here’s a map.) It contains a section of the Brule River, which drops a total of 800 feet in elevation on its trip through the park, creating waterfalls throughout the park’s landscape. Most of the Brule River ends up emptying, at the river’s mouth, into Lake Superior. Where does the other part go?

No one knows.

As pictured above, one of the Magney State Park waterfalls is, actually, two waterfalls in one. At the top, the Brule River splits into two parts, with the larger part (the eastern one) falling 50 feet in two steps, and continuing on its path to Lake Superior. The western half, however, falls 10 feet into a pothole called the Devil’s Kettle, for good reason: from there, no one knows where the water ends up. Researchers have attempted to determine where, exactly, the water comes out, by dropping ping pong balls, colored dyes, and other things into the water, but nothing ever re-emerges. And over time, many tree branches, rocks, and similar naturally-occurring objects have fallen into Brule River and fallen into the Devil’s Kettle, but there has not yet been located an area with a disproportionate deposit of these items lying around.

One theory holds that there are lava tubes — long, cavernous channels created from underground lava flows — running beneath the park. The problem with this theory: the rock type in the park, rhyolite, has never been known to feature lava tubes before. The more likely explanation is that the water makes its way into the area’s groundwater system, at some point finally emptying into the lake; or perhaps, the water rejoins the river here and there. But the belief is more a guess than anything else, based on the theory that the sheer amount of water being poured down the hole needs to come out somewhere, and is not simply collecting underground.

But for now? We’d advise not putting anything valued into the Kettle.

Bonus fact : In March, India’s space program identified what is believed to be a lava tube on the moon. While not the first one located, it may be the largest, extending over a mile long and nearly 400 feet wide. It is protected from the moon’s surface by a 40 foot thick wall, which, as The Week notes, makes it a good candidate for a future lunar settlement (!). Why does the wall matter? Because the moon doesn’t have an atmosphere which regulates the temperature on its surface; temperatures can swing wildly and well outside the ranges of human habitability (plus or minus 250 or so degrees Fahrenheit). The wall keeps the cave at -4 degrees Fahrenheit — and also protects this lava tube from meteorites.

From the Archives: Rainbow River: A technicolor river like no other (but we know where it goes).

Related: Want a waterfall of your very own? It can be yours — for just over $80.