Another theory blames the anomalous gravity on glacial rebounding, which occurs on much shorter time scales. During the last ice age, the two-mile-thick Laurentide Ice Sheet stretched from the Arctic down through eastern Canada to the northern half of the Midwestern United States, spanning 5 million square miles. The massive sheet pressed down on the Earth, deforming the crust somewhat like a Sumo wrestler on a trampoline surface.

Even though the icy cover has all but vanished, the Earth still feels the burden and like a slowly rebounding memory-foam pillow, it has yet to snap back to its ice-free shape. The gravity measurements reveal that the slight deformation could explain about 25 to 45 percent of the unusually low gravity that has persisted over a large section of Canada.