Sean Smyrichinsky thought he saw a UFO when he encountered object that may have been abandoned by an American bomber before crash in British Columbia

The Canadian navy will be heading to the coast of British Columbia to investigate claims that a diver may have come across “the lost nuke” – a Mark IV bomb that went missing after an American B-36 bomber crashed in the region during the cold war.

Diver Sean Smyrichinsky was wrapping up a day of diving near Haida Gwaii, an archipelago 80km west of the coast of British Columbia, when he stumbled across what may be the remains of the world’s first known “broken arrow” – the code name for accidents involving American nuclear weapons.

“I was just looking for fish for the next day. I figured I would do a little reconnaissance dive looking around and on my dive I got pretty far from my boat,” he told the Canadian Broadcasting Corporation. “And then I found something that I had never, ever seen before.”

The object was huge, he said, measuring around 12 feet long. “It resembled a bagel cut in half, and then around the circle of the bagel these bolts all molded into it, like half spheres. It was the strangest thing I had ever seen.”

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He came out of the water, excitedly describing the bowl-shaped object and its bolts that were bigger than basketballs. “I started telling my crew: ‘My God, I found a UFO.’” He sketched a rough outline of what he had seen on a napkin.

Smyrichinsky started asking around, curious if anyone else had ever come across the mysterious object. “Nobody had ever seen it before or heard of it. Nobody ever dives there,” he told the Vancouver Sun. “Then some old-timer said: ‘Oh, you might have found that bomb.’”

It was a reference to the Mark IV, a 10-foot, blimp-shaped nuclear bomb weighing some five tonnes and which went missing over the Pacific during a US air force B-36 training flight on 13 February 1950.

According to the Royal Aviation Museum of Western Canada, the intercontinental bomber had left an air base in Alaska for a mission that included a simulated drop on San Francisco when three of the plane’s six engines caught fire.

The crew was forced to abandon the bomber but US air force reports said they first jettisoned the bomb over the Pacific. The US military said the lost bomb was a dummy capsule – packed with lead rather than the plutonium core needed for an atomic explosion.

The bomber disappeared from the radar screen just before midnight. Days later, 12 of the 17 men onboard were found alive. The plane, set to autopilot by the crew before they parachuted out of the aircraft, crashed into the snow-covered mountains of northern British Columbia.

The parallels between what he had seen and the story of the lost nuke sent Smyrichinsky searching online. “And sure enough, there was a story about this lost bomb,” he told CBC.

An image turned up a photo resembling what he had seen. “A big circle with these balls, I had no idea that particular bomb contained all these big balls, bigger than basketballs.” A further search suggested that the balls – each some 20 inches across, he said – were home to the explosive stored in the Mark IV.

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The bomber had crashed some 50 miles south of where he had been diving. “I’m right in the right area and it looks like it could be a piece of that thing,” he said. “What else could it possibly be? I was thinking UFO, but probably not a UFO, right?”

Smyrichinsky detailed his find in an email to Canada’s department of national defence, who told him they were looking into the matter with “keen interest”.

The Canadian Armed Forces said on Friday that a Canadian navy ship would be deployed in the coming weeks to investigate the object. Government records indicate that the lost bomb was a dummy and poses little risk of nuclear detonation, said a spokesperson.

“Nonetheless we do want to be sure and we do want to investigate it further,” he said. A team specialising in unexploded ordnance will determine what risk, if any, the object poses and whether it should be retrieved from its resting place or left as is, he added.