Beluga whales aren't hiding legs under their thick, rubbery skin. But a popular image of a surfacing beluga has for years created the curious illusion of a whale with legs and knees.

The image, which excites the web from time to time, has recently turned up again. These symmetrical, bone-like features surely aren't legs, but they do serve an important purpose for the marine mammals, which naturally reside in frigid Arctic and sub-Arctic waters.

It's blubber. Specifically, long "rails" of extra fatty insulation the mammals store on their sides.

"It was just a weird camera angle," Carey Richard, the supervisor of cetaceans and pinnipeds at the Mystic Aquarium in Connecticut, said of the illusion.

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"The position of the camera is just such that they caught that blubber moving," said Richard, who recently helped rescue a stranded beluga whale calf in Alaska.

"I’ve never seen blubber looking like human anatomy," she added.

This insulating blubber accounts for around 40 percent of a beluga's body weight, said Richard, so it can be quite prominent along the sides of the swimming whales as they move around, she said.

And in this case, movement is almost certainly responsible for the "leg-like" illusion.

"My guess would be just the angle and movement of the animal through the water is causing this distortion in appearance of the blubber and maybe muscle," Mandy Migura, a National Oceanic and Atmospheric Administration marine mammal biologist, said via email.

There's an idea floating around the web that perhaps early seafarers mistook these belugas for mermaids. But this is almost certainly a tall tale, and makes little sense (as mermaids don't have legs, and also don't exist).

Richard has never heard the mermaid connection, but she suggested a far more plausible idea. Like many whales, belugas sing under the ocean, and Richard said belugas are particularly admired vocalists, known as "the canaries of the sea." Perhaps, then, voyagers in the northern seas, resting in their rocking, wooden-hulled ships, mistook the natural song of belugas for mythical, and also legless, mermaids.