the cavern had been sealed were undisturbed, but in the side of the hill was set a steel door beside the concrete. There was no sign of a keyhole or other means of entering it.
"Was this steel door part of your work?" asked Carnes.
"No, sir, it wasn't. We sealed it solid. That door has been put there since."
Dr. Bird closely examined the structure. He tapped it and went around the edges and then straightened up and took a small pocket compass from his pocket and opened the case. The needle swung crazily for a moment and then pointed straight toward the door.
"A magnetic lock," he exclaimed. "If we could find the power line it would be easy to force, but finding that line might take us a week. At any rate, we have found out what we were after. This is their base from which they are operating. Mr. Harron, I want you to station a guard armed with rifles at this door day and night until I personally relieve you. Remember, until I relieve you, in person. Verbal or written orders don't go. Capture or kill anyone who tries to enter or leave the cavern through this entrance. Just now we'll find that cavern more vulnerable from the sea end, and that is where I mean to attack. We'll force that door and explore from this end later. Commander Minden, you may stay here with Mr. Harron, if you like, or you may come with Carnes and me. We are going on board the Minneconsin."
THE Mitchell Field plane roared to a take-off and bore south along the coast. Half an hour of flying brought them in view of the battleship steaming at full speed up the coast. Dr. Bird radioed instructions to the ship, and an hour later a launch picked them up from the beach and took them out. As soon as they were on board they resumed their progress, and in two hours the peak that Dr. Bird had marked as a landmark was opposite.
"Steam in as close to the shore as you can safely," he said, "and then lower us. Once we are down, you will be guided by our telephoned instructions. Come on, Carnes, let's go."
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
