He twisted around, found the circular port through which he had originally entered the rocket. He worked at it with his fingers, realizing that it might be quite difficult to open. He worked away the padding that lined the interior, found that it had an arrangement that had automatically sealed it when closed. There was no handle on the inside, for it had never been planned to be opened from that side. However, there were several screws over a small plate, and Robin set to work to unscrew them. He had a Boy Scout knife in his pants pocket--the kind with several blades--and with the back of the biggest blade he worked out the screws. The panel off, he saw how the sealed gimbals worked, clicked them open and pushed open the door. It held tight for a moment, then popped open. There was a sudden drop in the pressure, Robin's ears popped, and he gasped for breath. The air outside was lower in pressure than that inside the cargo nose of the rocket, which had been sealed at Earth level. But it was air and it was breathable. Robin drew in several deep lungfuls, savoring it. It was oddly exhilarating, as if highly charged with oxygen. At the same time there was a smell of mold and dampness and a definite taste of sulfur and phosphorus like that just after a kitchen match has been lighted. Even so, the air was breathable. Robin worked his head and shoulders through the narrow opening, slid forward and landed on hands and knees on the rocky surface. He got to his feet, looked around. He was standing on the bank of a rushing stream of water, which was pouring out of a large gap in the side of a cliff. The cliff ran straight up, gently curving to form part of the ceiling several hundred feet overhead. The extent of this ceiling was impossible to determine--it was dark and obscure--but it seemed to Robin almost at once that he was in some sort of gigantic enclosed space--a vast cavern beneath the surface of the Moon, probably several miles beneath it. The water coming from the underground falls rushed out to form a wide, shallow river which flowed along one side of the cavern and widened out to a few hundred feet clear across to the farther wall. On Robin's side the floor of the cavern rose in a slow slope until it reached its wall perhaps three hundred feet away. Robin could not estimate the length of the cavern. Looking along the river bank, the cave seemed to become veiled in a general mistiness and gathering darkness. The light itself came from no definite source, but seemed to emanate from the rocky walls and ceiling, from the clayey ground, and from the general atmosphere. Robin supposed that the source was a natural phosphorescence which he knew was not too uncommon even in Terrestrial caverns.