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(MODEL RELEASED IMAGE). The Madsen family on a day of dogsled travel. When the snow crust is hard enough to ensure that the dogs won't break through, they can pull the half-ton weight of the sled for hours on end. On level ground, the animals pull at about the pace of a running human, but the sleds can whip down hills so fast that drivers must step on the brake at the rear of the sled to avoid running over their dogs. The dog sled is on flat sea ice here: a giant iceberg is in the background at the ice edge.  (Supporting image from the project Hungry Planet: What the World Eats.)