"Do you mean the one where they used to smuggle aliens? That is closed up. I was in charge of that work and we closed it tight as a drum two years ago." "Can you find the entrance?" "Sure. It isn't over a mile from here." "Lead the way, then. We want to take a look at it." The marshal led the way toward the eminence and took a path which led up a gully in its side. He paused for a moment to take his bearings and then turned sharply to his left and climbed part way up the side of the ravine. "Here it is," he announced. An expression of astonishment crossed his face and he examined the ground closely. "By Golly, Doc," he went on as he straightened up, "this place has been opened since I left it!" * * * * * Dr. Bird hurried forward and joined him. The heavy stone and concrete with which the entrance to 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."