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.
VIDEO: Man Survives Avalanche with Help of Fellow Snowmobilers
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.
ANALYSIS: Goldilocks-Like Clouds Melted Greenland Glaciers
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.