In the wild, catfish lead elusive lives, evading detection from scientists hoping to learn more about their lifestyles. And so, when police received reports of weird catfish clumps at the bottom of a waterfall, they immediately contacted a team of scientists to do some detective work.

After watching the fish for about 20 hours, the researchers arrived at the conclusion that these Rhyacoglanis paranensis—an orange-and-black species of bumblebee catfish common to South America—were scaling steep, vertical waterfalls as a team. A paper published August 8 in the Journal of Fish Biology records this bizarre yet remarkable moment.

“I would say it was a delightful surprise to all of us, crazy about fish, to witness all the tiny bumblebee catfishes gathering in a huge effort to climb the rocky walls—a great moment!” Manoela M. F. Marinho, study lead author and a biologist at the Federal University of Mato Grosso do Sul, told Gizmodo in an email.

The secret life of catfish hikers, explained

The catfish started their climb after 6 p.m., gathering by the thousands at the bottom of small waterfalls. The steeper the climb, the more the fish appeared to wiggle solo; some even slithered upside down across the ceilings of crevices. More teamwork was on display near flatter rocks, where the catfish climbed on top of each other to get over the rock.

The researchers aren’t entirely sure why the fish are engaging in this behavior. Based on dissections and the timing of the sighting, they believe the catfish were likely migrating upriver to reproduce. They also didn’t appear to have eaten. But this behavior does appear to be instinctual: “They also climbed artificial objects, such as our plastic pail,” the paper reports.

The study demonstrates the value of field observations in ecology and how chasing down strange reports can lead to valuable and unexpected discoveries. 

Personally, I’ve always thought catfish were cool. There’s something oddly charming about their flat, sausage-like build. And now that we know they’re good hikers? Even cooler.