At the end of one of the passages a large tank, perhaps ten feet in diameter, was sunk flush in the loose rubble. A circular iron plate in its top, sucked down by the inexorable magnetism, required the combined efforts of four of the sub-men to remove. The plate at last dragged aside, they motioned their two captives forward. For just a moment Haller hesitated, but with an atomite gun digging into his back, there was no choice. Gripping the edge of the opening he lowered himself into the tank. The drop of about six feet was jarring and he had just time to move aside as Barger landed beside him. A moment later the glowing sub-men had dragged the magnetized iron plate over the opening. * * * * * The interior of the metal tank that served as their prison was dark, except for a faint greenish fluorescence, like that which emanated from the renegade earthmen, visible in one corner. Moving toward it, Haller saw a copper vessel filled with water, apparently for the use of prisoners. "Barger!" he exclaimed. "That's why they give off that green light! It's the water! Phosphorescent water! We've seen it on earth often, caused by microscopic animal life! Only this is so full of the stuff that by drinking it, a living person becomes phosphorescent also! Like the deep-sea fish on earth! The human body's over eighty per cent water, remember!" "Interesting," the old quartermaster grunted, biting off a quid of blue Jovian _tole_. "But hardly helpful." He spat noisily. "What next?" Haller disregarded the question. "I'm beginning to get a clear picture of this," he announced. "For millenniums this little asteroid drew about it ferrous meteors. Then, two hundred years ago, man perfected the spaceship. Since then, this has become the Isle of Lost Spaceships. Hundreds of vessels, venturing too near, were caught in the field, drawn down. I can imagine the men on the first ship, half-mad, starved, before another was drawn down by the field, plundering the new arrivals of their food and supplies, killing their crews. Orth mentioned the _Transvalia_. She was the ship chartered by some fanatical religious sect who were going to found a new world. Also, she had women aboard. That was the start of this degenerate race. Two centuries of savagery, piracy, and we've seen the result." He paused grimly. "They're strong but stupid. That's our only chance. Also we haven't drunk any of this water and aren't fluorescent. That means we've a good chance of getting by unseen in these caves, once we get out of this tank." "And then, I suppose," Barger grinned, "we build an aluminum spaceship that isn't affected by magnetism and take off. Or do we thumb a ride on a comet?" By way of answer Haller commenced to examine their prison. A large cylinder, of a bronze-like alloy, it had no openings except the one at the top, covered by the steel plate.