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The first public scientific evidence occurred in 1947 when Rear Admiral Richard E. Byrd of the United States Navy flew directly to the North Pole and instead of going over the pole, actually entered the Inner Earth.
A crevasse is a deep crack, or fracture, found in an ice sheet or glacier, as opposed to a crevice that forms in rock. Crevasses form as a result of the movement and resulting stress associated with the shear stress generated when two semi-rigid pieces above a plastic substrate have different rates of movement. The resulting intensity of the shear stress causes a breakage along the faces.