In a strange, underwater video, a black mass drifts toward the camera. It's made up of a dark, spherical blob up front and a long, thin tail in the back.

"What is that?" a voice asks.

"Oh, wow," says another.

"Looks like a Muppet," says a third.

As the camera, mounted on an undersea, remotely piloted vehicle, approaches the mass, its face resolves. Big, googly eyes stare out from the front of the sphere, appearing, indeed, Muppet-like. A rigid line encircles the sphere about its equator. More sounds of approval come from the explorer team on board the research vessel Nautilus.

The camera gets a bit closer, and the sphere becomes something else: a writhing, amorphous shape, like a black rubber ball — or a ball of some more-uncanny material — trying to contain a tornado inside itself, inflating all the while. [10 More Fascinating Sea Creatures]

"Oooo-OOOH!" several voices exclaim at once.

"It's in full defense," says another.

As the strange creature turns in the water, writhing itself in a circle so that its waifish behind is presented to the camera, another voice agrees, saying, "That's his defense. Let me blow up, so I can show them how big I am."

Shaking violently back and forth, and turning back around toward the camera, the spherical mass splits along that rigid line. And the line is revealed to be the creature's jaw, with a gaping, diamond-shaped mouth opening across it. The being grimaces, horribly, for a moment as the sphere deflates. And the normal, thin body shape of the eel is revealed for viewers.

A moment later, the long edges of that jaw tuck back against the eel's body, disappearing like the wings on a fighter plane. Except for the color, tail and ongoing violent shakes of its head, the now-slimmed creature is almost unrecognizable as the same animal seen previously. The observing biologists make appreciative noises.

According to the Ocean Exploration Trust, the scientific nonprofit behind the Nautilus mission, the animal captured in this video was a gulper eel (Eurypharynx pelecanoides), also called an "umbrella-mouth gulper eel" or "pelican eel." Despite its fearsome appearance, the creature in this video was likely a juvenile, the researchers wrote in a press release emailed to Live Science. Adult eels of this species can grow up to 3 feet (nearly a meter) long.

The video was captured, the scientists wrote, as part of an expedition to document unseen regions of the Papahānaumokuākea Marine National Monument, an underwater site that extends northwest of Hawaii in the north Pacific Ocean.

Originally published on Live Science.