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:
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Bow-Nessie
In England, a kayaker took this photo on Lake Windemere, near Bowness in Cumbria. "At a distance, I thought it was some sort of large dog," said Tom Pickles. "Then I realized just how long it was." Ever since the first reports of Bow-Nessie emerged in 2006 from Lake Windermere, a legend has taken root with people wondering if this could be a not-too-distant relative of the legendary Loch Ness Monster of Scotland.
Loch Ness Monster
A view of the Loch Ness Monster, near Inverness, Scotland, on April 19, 1934. The photograph, one of two pictures known as the "surgeon's photographs," was allegedly taken by Col. Robert Kenneth Wilson, though it was later exposed as a hoax by one of the participants, Chris Spurling. On his deathbed, Spurling revealed that the pictures were staged by himself, Marmaduke and Ian Wetherell, and Wilson. References to a monster in Loch Ness date back to St. Columba's biography in 565 A.D. More than 1,000 people claim to have seen "Nessie," and the area is, consequently, a popular tourist attraction.
Loch Ness Sighting
This is a photo of boats at Urquhart Bay, Loch Ness, Scotland, on Aug. 6, 1983, made by American wildlife photographer Erik Beckjord. It shows splashes on the surface of Loch Ness made by an unidentified object (white mark at center right), which Beckjord claimed could have been made by the Loch Ness Monster.
Loch Ness Monster Fin
Robert H. Hines, president of the Academy of Applied Science, released this photo during a 1972 investigation of Loch Ness. Hines said his expedition took photo, which he said showed the fin of the Loch Ness Monster, and that it was substantiated by sonar and other scientific data that strongly suggests there is a large marine creature inhabiting Scotland's Loch Ness.
Mersey Monster
This mysterious shape was captured by photographer Mark Harrison while riding on a ferry off the Seacombe district of Wirral in the United Kingdom on the morning of May 25. Experts claim that it could be a harbor porpoise or a basking shark, but Harrison says, "Me? Clearly, I believe it's Nessie on her hols!"
Nessie and Alaska's Illie May Be Sleeper Sharks
This 2000 image shows biologist Bruce Wright in salt waters in southeast Alaska with a small Pacific sleeper shark that was caught on a research cruise. He believes much bigger versions of this shark group could be the true identity of Scotland's Loch Ness Monster and Alaska's Lake Iliamna creature known as Illie.
Dorset Pliosaur
The giant jaws of a huge marine reptile are on permanent display at Dorset County Museum in the UK. Dating back around 155 million years, the pliosaur skull was discovered on the nearby Jurassic Coast World Heritage Site, and is one of the largest and best preserved fossils of its kind ever found.
Chupacabra
Phylis Canion holds the head of what she called a Chupacabra at her home in Cuero, Texas, on Aug. 31, 2007. She found the strange-looking animal dead outside her ranch and thinks it is responsible for killing many of her chickens.
Montauk Monster
Was it a dog or a pig or something else? Nobody knows for sure, but this animal was discovered in 2009 on a beach in Southold, on the North Fork area of New York's Long Island. Subsequent photos were published on <a href="www.montauk-monster.com" target="_hplink">Montauk Monster</a>.
East River Monster
This Atlantic sturgeon corpse measuring more than 6 feet long was pulled from the East River in New York City near Pier 17 on May 21, 2011. <a href="http://www.ny1.com/content/139542/mystery-east-river-fish-identified-as-sturgeon" target="_hplink">Watch video here.</a>
Bigfoot in Rutherford County, N.C.
Thomas Byers snapped this photo of "Bigfoot" along Golden Valley Church Road on March 22, 2011.
California Bigfoot
This is a 1977 still photo from a 16 millimeter film made by Ivan Marx reportedly showing the legendary Bigfoot cavorting in the hills of Northern California.
Washington State Sasquatch
This is a black and white print from a color movie Frank White said he took in the forest near Bellingham, Wash., on Oct. 8, 1977. "I'd call it a North American ape," said White. "You can call it a Sasquatch or anything you like."
Georgia Bigfoot
This still-frame image from video provided by Bigfoot Global LLC shows what is claimed by them to be a Bigfoot or Sasquatch creature in an undisclosed area of a northern Georgia forest in June 2008.
Bigfoot Footprint
The legend of Bigfoot has baffled many people for decades, especially when images like this one are released. A footprint measuring 17-and-3/4 inches long and 7-and-1/2 inches wide was discovered Aug. 26, 1980, at a residence in the Conemaugh Township area of Johnstown, Pa. A very well-defined print was left behind, if indeed it was Bigfoot, plus a left print was found eight feet away in a more wooded area. Along with the footprints, reports of strange noises and a very unusual but strong odor coincided with the account.
Bigfoot Plaster Cast
Ken Gerhard of Houston holds a duplicate plaster cast footprint at the Texas Bigfoot Conference in Jefferson, Texas, on Oct. 15, 2005. The event was hosted by the Texas Bigfoot Research Center.
Bigfoot Casts
Al Hodgson, a volunteer guide at the Willow Creek-China Flat Museum, holds up a plaster cast of a Bigfoot imprint displayed at the museum's Bigfoot Wing in Willow Creek, Calif., on June 5, 2000. The wing was built to bring tourists interested in the legendary creature to the economically depressed ex-lumber town.
Bigfoot Body?
This photograph obtained Aug. 15, 2008, from <a href="www.searchingforbigfoot.com" target="_hplink">Searching For Bigfoot</a> shows what is purported to be the body of the legendary ape-like creature that has been the subject of decades of hoaxes and dubious sightings. Matthew Whitton and Rick Dyer claimed before a crowd of skeptical reporters in Palto Alto, Calif., that they were hiking in a northern part of Georgia when they stumbled upon the body near water. The corpse was said to be 7 feet, 7 inches tall, weighing more than 500 pounds. Many scientists believe Bigfoot is folklore instead of fact.
Abominable Snowman Footprint
This alleged Abominable Snowman footprint photo was taken near Mount Everest on Dec. 13, 1951.
The Jersey Devil
An artist's rendition of the Jersey Devil, based on eyewitness reports of a creature said to roam the Pine Barrens area of New Jersey.
Mothman
A large Mothman sculpture stands along Main Street on Dec. 11, 2007, in Point Pleasant, W.Va. More than 40 years after the first reported sighting of the mysterious creature later dubbed "Mothman," residents here have embraced his legend, helping to turn the town into a destination for people in search of an offbeat tourism experience.
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First Posted: 07/20/11 08:03 PM ET Updated: 09/19/11 06:12 AM ET