At first, the researchers monitoring a deep sea dive thought the sea floor was moving with sediment. But on closer inspection, they found something completely surprising: a carpet of red crabs, swarming by the thousands.

Dr. Jesús Pineda, a biologist at Woods Hole Oceanographic Institution, was on the team that was searching for clues as to why volcanic sea mounts tend to attract a large diversity of marine life. While they witnessed quite a few expected species, they were stunned to see thousands of crabs moving together along the sea floor.

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

This swarm behavior is a first of its kind documented for the red crab, and the reason behind the behavior is still a complete mystery.

Also surprising was where the crabs were swarming, since they are typically found quite a bit farther north. "No one had ever found this species that far south," Pineda said. "To find a species at the extreme of their range and to be so abundant is very unusual."

The video below is interesting because it sets up a context for what the scientists saw. But if you just want to see the crazy carpet of crabs, skip to about 1:10 in the video.

Hannibal Bank Seamount Expedition from Woods Hole Oceanographic Inst. on Vimeo.

Not long after the swarm was sighted, thousands of red crabs washed ashore in San Diego.

"The researchers wondered if they were the same species as the Hannibal red crabs. After obtaining samples from a beach in San Diego in June 2015, DNA analysis revealed that the Panamanian and the Californian crabs are in fact the same species," reports WHOI.

As Pineda notes in the video, this discovery highlights why exploring the sea is so critical, particularly exploring it with our own eyes. Just as no one could have predicted the when, where and what of the swarm of crabs, there is much we don't know and can't know about the ocean unless researchers get under the water and look around.

Only a tiny fraction of the ocean has been explored, and relatively little is known about the abundance of life that calls the ocean home.

Jaymi Heimbuch ( @jaymiheimbuch ) focuses on wildlife conservation and animal news from her home base in San Francisco.

Swarm of crabs on the ocean floor startles researchers
What looked like a cloud of sediment turned out to be thousands of red crabs moving together.