A narrow steel ladder led down into the cylinder. In the darkness, its slender rungs offered tricky footing, but the two Navy men made short work of the descent. Thirty feet below the hatchway, they reached a dimly lighted landing, from which two doors opened.

“Take the berth deck, Red,” Don directed curtly. “Get Yanos and the two native fishermen out of their hammocks and up the ladder. I’ll bring Jerry from the chartroom. If he’s still unconscious I’ll carry him top-side.”

“Aye-aye, Skipper!” muttered Red Pennington, pushing through the left-hand door. “If you need any help, just sing out!”

A short corridor led Don Winslow to the cylinder’s crowded chartroom, where the seaman, Jerry Ward, lay on a cot between two banks of electrical apparatus. Don glanced with envious eyes at the array of super-sensitive instruments.

“If only we had time to get some of this stuff aboard the gunboat!” he muttered. “No time to think about that now, though. That plane overhead may lay an 'egg’ on this place any minute!”

Bending over the unconscious Jerry, he shook the man gently. There was no response. A head wound, received at the time of his capture, had left the plucky fellow hanging between life and death.

Carefully Don lifted the limp body in his arms and turned to the door. As he did so, a muffled explosion shook the steel walls about him.

Bursting out onto the lower landing, Don Winslow collided with Lieutenant Pennington.

“Quick, Red!” he barked. “Take Jerry on your back, and get up that ladder. I’ll lash his wrists together, so you’ll have both hands free to climb with. Where are Yanos and the others?”

“They’ve just gone up!” Red answered, stooping to take Jerry’s weight. “And say! That did sound like a bomb overhead, just now! We’d better get out of here in a hurry!”