A nine-year-old boy fell into a hole in a glacier while riding a snowmobile during a sporting event in Alaska on Saturday, reported the Fairbanks Daily News Miner. Alaska State Troopers believe the boy died in the fall and is buried in deep snow at the bottom of the hole, known as a moulin.

A moulin forms when melt water on the surface of a glacier eats a hole into the ice. The well-like tunnels created by moulins can bore hundreds of feet down into the glacier.

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Although deadly deep, the relatively narrow mouth of a moulin can become nearly invisible when covered by a bridge of snow. To spot a moulin, mountaineers and skiers must watch for areas where the snow appears to be sagging, according to Jason Martin of the American Alpine Institute in a blog. He recommended that mountaineers be constantly ready for a moulin or crevasse and that adventurers tie themselves to their companions with rope to ensure safety.

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On a snowmobile, the subtle signs of a moulin can be very easy to miss. The unfortunate boy in Alaska may not have seen any sign of danger.

The danger of snow bridge trickery increases in early spring in a glacier’s accumulation zone, according to Backpacker magazine.

IMAGE: Moulin at the Athabasca Glacier (China Crisis, Wikimedia Commons)