The Brule river in Minnesota splits into two waterfalls as a result of an outcrop of rhyolite rocks. One waterfall feeds Lake Superior. Geologists aren’t sure where the water in the other waterfall ends up.
https://www.mnn.com/lifestyle/eco-tourism/stories/the-mystery-of-devils-kettle-falls
It’s called “Devil’s Kettle Falls,” and some might say the Devil’s at work here, but geologists continue to employ science in their search for the other half of the waterfall. The reason we can’t just observe where the water goes is that it descends into a large cavernous depth called the “Devil’s Kettle.” The obvious guess is that the water joins Lake Superior at some point. However, scientists haven’t been able to prove it. They’ve tried pouring dye and throwing logs into Devil’s Kettle, but no signs of these things have later been seen in Lake Superior. Scientists plan on using a dye trace method to follow the water trail during low-volume periods to figure out what’s really going on. An alternate theory suggests that the water siphons off into an ancient subterranean lava tube.
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