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A mysterious cloud of disturbed silt at the Hannibal Bank seamount off the coast of Panama has led marine biologists to an awesome, and unexpected, sight: a deep-sea red crab invasion near the sea bottom that the research team describe as like a swarm of insects.

The scientists, led by Jesús Pineda from the Woods Hole Oceanographic Institution, were in a submersible investigating biodiversity at the underwater mountain, which is known as an ecological hotspot, when they came across the teeming mass of crabs at a depth of around 1,200 feet. The highest density measured was an astonishing 78 crabs in a square meter (roughly seven crabs per square foot). Pineda comments: “It was quite unexpected. I didn’t know that these swarms existed.”

DNA tests of a few collected specimens later revealed the species as Pleuroncodes planipes, which is abundant off the coast of Baja California. Around the same time the video was shot, April last year, the crabs washed up in incredible numbers on beaches further north in San Diego, which has been put down to the current El Niño and western Pacific waters being warmer than average.

The reason for the swarm behavior remains a mystery, and Pineda doesn’t think migration, feeding or mating explains it, given that the animals are all mysteriously moving in the same direction (see video below). The team intends to return to the area in an effort to understand the role the crabs play in the productivity of the seamount.