Already great seas were booming and breaking with a roar upon the ice, and as the boys turned the dogs back upon the trail they observed a waving motion of the ice beneath them, which was rapidly becoming more apparent. At one moment the dogs would be hauling the sledge up an incline, and at the next moment the sledge would be coasting down another incline close upon the heels of the team, as the heaving ice assumed the motion of the seas which rolled beneath.