On the Brule River in Minnesota’s Judge C. R. Magney State Park, an unusual waterfall has sparked imaginations for decades. That’s because this waterfall, called the Devil’s Kettle, splits into two: one half of the river flows up to the edge of the falls and tumbles over, but the other half slips into a hole at the top of the falls and disappears.

Recently, scientists were able to solve the mystery of the disappearing side using a simple stream-gauging technique. It turns out, the water doesn’t disappear at all.

“People thought maybe it went to Canada because you're not very far from Canada, or that it went out into Lake Superior and came up like a submarine spring,” Jeff Green, a groundwater hydrologist with the Minnesota department of natural resources, told Motherboard’s podcast Science Solved It. “There were just various theories like that, that the water was going away from the river. That was the key—that half the river flow was leaving the river.”

Locals in the area claimed that everything from sticks to ping pong balls, road signs to even a car, had supposedly been dropped into the hole over the years and never re-emerged. Others claimed a young man had rappelled 26 feet into the hole and never saw the bottom.

The assumption was that the water split in two and some portion of the river water was leaving the main stream and being diverted elsewhere. Green wasn’t convinced.e wanted to test the theory before trying to solve where the water was going, so he had a state water gauging crew measure the water flow above the falls and right afterwards to see if any of the water actually left the river once it disappeared into Devil’s Kettle.

“A few weeks later they pulled things together and sent them to me and said, ‘Here's the numbers: 123 cubic feet per second upstream from the Devil’s Kettle [and] 121 cubic feet per second downstream,’” Green said. “In the world of stream gauging, those are basically the same number. They're within a few percent error, which is very, very good.”

The measurements showed that none of the water was actually leaving the river, which meant all of the flow that tumbled into the Devil’s Kettle and seemed to disappear actually rejoined the stream at the base of the falls. Green told me they don’t know exactly where it re-emerges, but you can see two distinct “boils” of water at the base of the falls. He suspects the Devil’s Kettle empties out right below where it flows in.

As for the ping pong balls and other detritus supposedly swallowed up over the years, Green told me that explanation was simple: they would have been smashed to smithereens before they ever got a chance to re-emerge.

“When water comes back up it recirculates. That's why dams are such drowning machines,” Green said. “If people dumped ping pong balls in they would be smashed flat by the recirculating currents as the water goes over [the falls].”