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."

"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