"Once we had the place spotted, we sent a gang in and captured the whole works without any trouble. The underground cavern had no natural opening to the surface, but one had been made by blasting. We captured the whole lot and then sealed the end of the hole with rock and concrete. That was the end of the affair." "Thank you, Commander; you have given us a very graphic description of it. I suppose you could find the entrance which was sealed up?" * * * * * "Easily. I led the raiding party. I forgot to mention one blunder we made. Evidently some word of our plans leaked out, for the sub which was guarding the outer end of the tunnel was called away by a radio message supposed to be from the Navy Department. It had gone only a short distance, however, when the commander smelled a rat and made his way back. He was too late. He was just in time to see the sub emerge from the hole and head into the open sea. He gave chase, but the other sub was faster than the Navy boat and it got clear away. The leader of the gang must have been on it, for we didn't get him." "Who was the leader?" "From some records we captured, his name was Ivan Saranoff. I never saw him." "Saranoff?" said Dr. Bird thoughtfully. "The name seems familiar. Where have I--Thunder! I know now. He was at one time a member of the faculty of St. Petersburg. He was one of the leading biologists of his time. Carnes, we've found our man." "If you are thinking of Saranoff, I am afraid you are mistaken, Doctor," said Commander Minden. "Neither he nor his submarine have ever been heard of since and it has been generally conceded that they were lost at sea. We had some pretty rough weather just after that affair." "Rough weather doesn't mean much to a sub, Commander. I expect that he's our man. At any rate, the place we want to go is the end of that tunnel." "I'm at your service, Doctor." "Carnes, get the location of that tunnel entrance from Commander Minden and order the _Minneconsin_ to proceed north along the coast to that vicinity and stand by for radio orders. I am going to telephone Mitchell Field and get a plane. We have no time to lose." * * * * * The plane from Mitchell Field roared down to a landing, and Carnes, Dr. Bird and Commander Minden dismounted from the rear cockpit and looked around. They had landed in a smooth field at the base of a rise almost rugged enough to be called a mountain. A group of three men were standing near them as they got out of the plane. One of the men approached. "Dr. Bird?" asked the newcomer. "I am Tom Harron, United States Marshal. These two men are deputies. I understand that I am to report to you for orders." "I'm glad to know you, Mr. Harron. This is Operative Carnes of the Secret Service and Commander Minden of the Coast Guard. We are going to explore an underground cavern that is located in this vicinity."