The detective followed him into the sphere as the _Minneconsin_ edged up toward the shore. The huge ball was lifted from the deck and lowered gently into two hundred fathoms of water. It was pitch dark at that depth, and Dr. Bird switched on one floodlight and studied the cliff which rose a hundred yards from them. "We have missed the place, Carnes," he said. "We'll have them pull us up a few hundred feet and then steam along the coast." He turned to the telephone and the sphere rose while the battleship steamed slowly ahead, the vitrilene ball following in her wake. For a quarter of a mile they continued on their way, and then Dr. Bird halted the ship. "What depth are we?" he asked. "Eighty fathoms? All right, lower us, please." * * * * * The ball sank until it rested on the sea bottom, and Dr. Bird turned on two additional floodlights and studied the surroundings. The bed of the ocean was literally covered with lobster and crab shell, with the bones of fish scattered here and there among them. A few bones of land animals were mixed with the debris and Carnes gave a gasp as Dr. Bird pointed out to him a diving helmet. "We are on the right track," said the scientist grimly. He stepped to the telephone and ordered the sphere raised to one hundred fathoms. The ship moved forward along the coast until Dr. Bird again stepped to the telephone and halted it. Before them yawned the entrance to the underground tunnel. It was about two hundred feet high and three hundred across, and their most powerful beams would not penetrate to the end of it. A pile of debris could be seen on the floor of the tunnel and Carnes fancied that he could see another diving helmet among the litter. Dr. Bird pointed toward the side of the cavern. "See those floodlights fastened to the cliff so that their beams will sweep across the mouth of the tunnel when they are lighted?" he said. "Apparently the cave is used as a prison and the light beams are the bars. The creature is not at home just now or the bars would be up. My God! Look at that, Carnes!" Carnes stared and echoed the Doctor's cry of surprise. Clinging to a shelf of rock which extended out from the wall of the cavern and half hidden among the seaweed was a huge marine creature. It looked like a huge black slug with rudimentary eyes and mouth. The thing was fifty feet in length and fully fifteen feet in diameter. It hung there, moving sluggishly as though breathing, and rudimentary tentacles projecting from one end moved in the water. "What is it, Doctor?" asked Carnes in a voice of awe.