The pressure was not steady, changing as the roaring itself changed, with sudden bursts of sound, convulsive shoves, and changes in pitch. The rocket was being slowed by a terrific outward burst of gases, gases that must have been imprisoned in a huge volcanic bubble whose outermost surface was the dome, so mysterious to Terrestrial observers. By bursting through the thin lava shell, Robin's rocket had released these pent-up gases and was boring its way down on its still rapid momentum against the pressure of this column of gas. Robin did not know this at the time, though he figured it out later. At the time, he had all he could do to keep himself from being battered black-and-blue by the jolting rocket. He kept his head clutched tightly in his arms, rode with the bumps and roars, and tried to keep his breath from being knocked out of his lungs. There was another violent shock and crack and again the rocket bounced to a new flow of gases. It had slammed through one huge bubble, breaking through the bottom shell only to burst into a lower pocket of gas. The roaring subsided to a lower pitch as the new gases did not find the near-vacuum of the surface that the first gas bubble had opened upon. The rocket fell steadily, bursting through a third, and then a fourth such bubble. It was clear that the surface of the Moon, at least in that area, was a mass of congealed gas pockets, a honeycomb of thin-walled lava bubbles, perhaps quite deep. The rocket was almost entirely devoid of its original space momentum by the time it hit the bottom of the last bubble, snapped the thin crust, and fell through it. This time there was a sudden hissing around the battered nose and a warmth began to flow through the body of the rocket. It was enveloped in a belt of hot steam through which it fell several hundred feet and then hit something with a loud splashing noise. The sound vanished as the rocket sank deep into the new substance, came to a halt, and bobbed back upward. Robin had gotten hold of himself after the third bubble and was hanging on, mentally trying to estimate what had happened. This last sound had been familiar. It must have been water, and the bobbing back of the rocket to the surface confirmed his views. He felt the rocket bounce a couple of times and then subside to a gentle rocking and rolling. Robin held on for a moment, getting his balance. In some ways the new motion was more disturbing than all that had gone before--the cylindrical body of the rocket, with its blunt end and its rounded nose, was twisting and turning as only can be done by a bottle tossed in a flowing stream. Robin tried to get hold of himself, orient himself to the odd seasick motion, then managed to work his way to the peephole.