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
oc