Researchers from the Woods Hole Oceanographic Institution came across a surprising sight while studying marine biodiversity off the coast of Panama: thousands of crabs swarming on the ocean floor in low-oxygen waters.

"When we dove down in the submarine, we noticed the water became murkier as we got closer to the bottom," said Jesús Pineda, a biologist at Woods Hole Oceanographic Institution and chief scientist on the cruise, in a release. "There was this turbid layer, and you couldn't see a thing beyond it. We just saw this cloud but had no idea what was causing it."

"As we slowly moved down to the bottom of the seafloor, all of the sudden we saw these things," says Pineda. "At first, we thought they were biogenic rocks or structures. Once we saw them moving—swarming like insects—we couldn't believe it."

The crabs — Pleuroncodes planipes — are abundant off Baja California, but have not been documented this far south until now. To see the behavior and learn more, check out the video above.