Small fishes swam past them along the passage, their round eyes peering in through the helmet glasses as they passed. Clumps of colored sea-weed grew out from the walls and ceiling, their long streamers waving gently in the slow currents set up by the passage of the men. In spite of the brightness of the light from the ceiling plates, the effect of the water made it difficult to see far down the passage ahead. The outlines of Slag were clear enough as he plodded along directly ahead of Gerry, but everything beyond him was a little blurred and uncertain. It was like living in a mirage. At last they came to a point where the passage branched. Here they passed a sentry who wore a glass helmet and a tight fitting green rubber uniform. On his chest was the insignia of a rampant black dragon. He was armed with a very thin, almost needle-like sword whose point was razor keen. Gerry realized the reason for that peculiarly designed weapon when the sentry swung his sword upward to salute their guide. The blade was so thin that it offered little resistance to the water, and its power of being quickly wielded made it a far more effective weapon under water than a heavier sword would have been. They passed more branching passages, and more rubber-clad sentries who stared at them curiously as they went by. There was a whole network of corridors in this underwater world! At last Slag opened a metal door at the end of the particular passage he had followed, and they all crowded into a small room. Slag closed the door and dogged it, then tapped on a glass panel across the room. A silvery flood of air bubbles came pouring out the end of a pipe that protruded through the wall. At the same time Gerry heard the thud of heavy pumps starting to suck water through gratings at the base of the wall. The water level dropped rapidly. When it was down to their waists, Slag took off his helmet and slipped the leaded sandals from his feet. He motioned to the others to do the same. "We are about to enter the hidden realm of Luralla, the home of the Dragon's Teeth!" he said. "If you can prove your right to be here you will be welcome. Otherwise you will go back into one of these waterlocks--without any helmets on." He grinned cheerfully. * * * * * The water dropped below the level of the door sills. The pumps sucked noisily on the last few foaming inches for a moment, and then they ceased. The inner door was opened by a sentry whose tight fitting green uniform with its black dragon was made of dry cloth instead of dripping rubber. He wore a plumed metal helmet, and carried a heavy sword instead of one of the thin water blades. "Come in, Slag," he said, "who are these strangers?"