Kelly Nash's Cadborosaurus footage, Nushagak Bay, Alaska

Cadborosaurus, known to its followers simply as Caddy, is the sea serpent of the Pacific Northwest. In the usual telling it has a long neck, a horse-like head, large eyes and bumps along its back that break the surface. Remains have come and gone. One carcass, found in a sperm whale's stomach at Naden Harbour in 1937, was photographed and later lost. A museum curator judged it a fetal baleen whale.

2009. Nushagak Bay, an arm of Bristol Bay in southwest Alaska. Kelly Nash, a commercial fisherman, filmed the water for several minutes.

By one description, the footage shows ten to fifteen creatures, some of them young. As one swims, a head pops up out of the water. Later it appears to blow something out of its head or neck. One report put the creature at 20 to 30 feet long. The footage is grainy, shot in the rain from a boat that would not keep still.

Tuesday, 19 July 2011. The Discovery Channel aired a short edited segment in a special called Hillstranded. Andy Hillstrand and Johnathan Hillstrand, captains from Deadliest Catch, watched the tape and set out to find the animals themselves.

They never found Nash's pod. But at one location where Caddy had been reported, they saw something. "We saw a big, long, white thing moving in the water," Andy Hillstrand said. "We chased it for about 20 minutes. Spray came out of its head."

It was definitely not a shark, he said. A giant eel did not fit either. The humps all moved in unison. He stood by the local fishermen. They "are not a bunch of fruitcakes."

The strongest reaction came from Paul LeBlond, former head of Earth and Ocean Sciences at the University of British Columbia and co-author of Cadborosaurus: Survivor from the Deep. "I am quite impressed with the video," he said. "Although it was shot under rainy circumstances in a bouncy ship, it's very genuine." The animal, he argued, "must be a mammal or a reptile, since it oscillates up and down in a vertical plain." Of known creatures, it was "the least unlike a plesiosaur."

LeBlond also said more physical evidence was needed, and that there would be no official hunt. John Kirk, president of the British Columbia Scientific Cryptozoology Club, called the video important.

Sceptics suggested a frill shark, a large eel or some other fish, but LeBlond rejected fish because of the vertical motion. Jim Covel of the Monterey Bay Aquarium said such reports rest on "sketchy lay observations." Several outlets noted that LeBlond was not a neutral observer. In 2012, Darren Naish argued more broadly that Caddy eyewitness reports lump together seals, swimming deer or moose, standing waves and lines of swimming animals. Hillstrand offered his own idea. Whales following salmon might be pushing the animals closer to shore.

In July and August 2011 the story travelled through NBC, Fox News, GMA News, Global News and papers as far away as Sri Lanka. The raw footage has never been made public. A fan-enhanced version circulates online. The identity of the animals in Nushagak Bay was never confirmed.