These little fish climb a 50 foot waterfall every year. Scientists still don’t know why.

Rumors have been circulating in parts of the Democratic Republic of the Congo for a while now about fish that swam—wait, no, climbed– up the waterfalls. It sounded like a legend or a lie. Or maybe just a misinterpretation of a real thing that twisted and mutated through a nationwide game of Telephone.

But it was true, and scientists finally got proof.

The species known as shellfish fish (Paracneria thysi) is less than two inches long. According to research published every year, at the end of the rainy season, thousands of them try to do something that sounds so stupid that it should have wiped out their species long ago: climb the almost completely vertical rock face of Luvilombo Falls in Upemba National Park. The climb is about 15 meters high and takes about 10 hours to complete. Most of the time is spent moving; it is used to rest. They pause for long stretches on small ledges, gathering in clusters along the way to take a breather.

They were able to climb a rock face thanks to specialized fins equipped with tiny hook-like structures that gripped the wet rock as they twisted up in short, twisting bursts. It’s a slow, tedious process, but an effective one.

Scientists first documented the behavior in 2018 and 2020, and thanks to a report published in Scientific reportswe are finally learning about it now.

The behavior appears to be part of a regular migratory pattern, as the fish begin to climb in April and May, starting in the early evening, possibly to avoid predators or to avoid being swept away by stronger currents earlier in the season. As for why they do it in the first place, all researchers can do for now is speculate. This could be avoiding local predators such as catfish or returning upstream after flooding? Nobody knows yet.

One thing is clear, illegal fishing in the region and diversion of water for irrigation threatens to disrupt the flow of this mountain-climbing fish.