The waves were splashing outside the shell. Thurston turned in a feverish search of the unexplored depths. There was a surprising simplicity, an absence of complicated mechanism. The generator, with its tremendous braces to carry its thrust to the framework itself, filled most of the space. Some of the ribs were thicker, he noticed. Solid metal, as if they might carry great weights. Resting upon them were ranged numbers of objects. They were like eggs, slender, and inches in length. On some were propellers. They worked through the shells on long slender rods. Each was threaded finely--an adjustable arm engaged the thread. Thurston called excitedly to the other. "Here they are," he said. "Look! Here are the shells. Here's what blew us up!" * * * * * He pointed to the slim shafts with their little propellerlike fans. "Adjustable, see? Unwind in their fall ... set 'em for any length of travel ... fires the charge in the air. That's how they wiped out our air fleet." There were others without the propellers; they had fins to hold them nose downward. On each nose was a small rounded cap. "Detonators of some sort," said MacGregor. "We've got to have one. We must get it out quick; the tide's coming in." He laid his hands upon one of the slim, egg-shaped things. He lifted, then strained mightily. But the object did not rise; it only rolled sluggishly. The scientist stared at it amazed. "Specific gravity," he exclaimed, "beyond anything known! There's nothing on earth ... there is no such substance ... no form of matter...." His eyes were incredulous. "Lots to learn," Thurston answered grimly. "We've yet to learn how to fight off the other four." The other nodded. "Here's the secret," he said. "These shells liberate the same gas that drives the machine. Solve one and we solve both--then we learn how to combat it. But how to remove it--that is the problem. You and I can never lift this out of here." His glance darted about. There was a small door in the metal beam. The groove in which the shells were placed led to it; it was a port for launching the projectiles. He moved it, opened it. A dash of spray struck him in the face. He glanced inquiringly at his companion. "Dare we do it?" he asked. "Slide one of them out?" Each man looked long into the eyes of the other. Was this, then, the end of their terrible night? One shell to be dropped--then a bursting volcano to blast them to eternity.... "The boys in the planes risked it," said Thurston quietly. "They got theirs." He stopped for a broken fragment of steel. "Try one with a fan on; it hasn't a detonator." The men pried at the slim thing. It slid slowly toward the open port. One heave and it balanced on the edge, then vanished abruptly. The spray was cold on their faces. They breathed heavily with the realization that they still lived. * * * * *