Making a quick decision, he began clearing rubble and sand away from the hole. After half an hour of this, the hole was no larger, but his conviction that it opened into a subterranean pit had become a certainty. Two small boulders, half buried, and adjoining the opening, were obviously jammed together by the force of too much mass crowding the mouth of a shaft; they seemed caught in a bottleneck. When he pried one stone toward the right, its neighbor rolled left, until no further motion was possible. The reverse effect occurred when he pried in the opposing direction, but he continued to jostle at the rock-jam.