It has a horse-like head at the end of a long neck, with big eyes and humps on its back. It undulates above the surface for just a moment before diving back underwater -- leaving even the most seasoned sailors scratching their heads.
It sounds like any of the thousands of sea serpent stories, from Scotland's Loch Ness Monster to Lake Champlain's Champ. But this time there's video.
A remarkable piece of footage purportedly shot by a fisherman in Bristol Bay shows what many people believe is Alaska's version of the Loch Ness Monster, reports Discovery News.
WATCH THE ALASKA SEA SERPENT VIDEO:
"These are large animals, swimming up and down and probably propelling itself by the flap of its tail. It's clearly not a whale and not a seal," said scientist Paul Leblond after viewing the video which premiered on this week's edition of the Discovery Channel's "Alaskan Monster Hunt."
"That looks a lot like Cadborosaurus," said Leblond, former head of the department of Earth and Ocean Sciences at the University of British Columbia.
In the last 200 years, numerous sightings of a Cadborosaurus -- named from Cadboro Bay in British Columbia and the Greek word "saurus" (or lizard) -- have come to light as people claim to see a serpent-like animal, reportedly up to 30 feet long and sometimes showing front and rear flippers. "Caddy," as the animal is nicknamed, is the subject of a book, Cadborosaurus: Survivor From the Deep, co-written by Leblond.
The Cadborosaurus, like most sea serpents reported around the world, is categorized by scientists under the heading of cryptozoology, the study of animals not yet recognized by mainstream science, like the Loch Ness Monster and Bigfoot.
While Caddy has been written off by some as a shark, eel or whale, Leblond disagrees.
"It must be a mammal or a reptile, since it oscillates up and down in a vertical plain, which eliminates sideways-oscillating fish," he said.
Whether Caddy or its Scottish cousin, Nessie, are relatives or even real, remains to be seen -- at least until more compelling visual evidence is presented. Or one of these beasts is actually captured.
MORE WATER BEASTIES AND OTHER CRYPTOZOOLOGY CREATURES: