The time charge exploded. The _NX-1_ rocked like a quivering reed; Wells was knocked violently to the floor; a vast roar smote his ear-drums. When he staggered to his feet he found that the octopus that was pulling at the charge had disappeared--blown into fragments of flesh and metal. But the hawser arm was broken! The _NX-1_, free, shot back a full fifty feet under the pull of her reversed screws. A cry echoed in her commander's ears: "Go back, Keith! Go like hell!" He saw the remaining octopus lift Bowman and whip to the exit port of its submarine. The lid slid into place, closing on the monster and his friend, and the enemy ship vanished into the black cavern.... * * * * * Once clear of the opening, Keith set his motors full forward and brought the diving rudders up. Quickly the ship sped from the haunted sea-floor to the sun-warmed surface. A last thin call rang in his radiophone: "They've got me inside, Keith. It's dark, and filled with water. I can't see anything, but I--I guess we're going through the cavern.... Forget about me, old boy. So long! So--" The voice was abruptly cut off. Keith ripped the instrument from his head. Then, face white and drawn, he ran to the radio cubby. Standing over Sparks' inert body, he put through a call to Robert Knapp, on the _Falcon_. "Knapp?" he said harshly. "This is Wells. I'll be with you in a few minutes. Yes--yes--I'll tell you the whole story later. But get this now: Have the day shift all ready to take over the submarine by the time I pull alongside." He said no more just then; but rang off, and, looking back, he muttered savagely: "But I'll be back, Hemmy--I'll be back!" CHAPTER IV _In the Cavern_ "That's the story, Knapp. They got Bowman, and I had to run away. Their ship disappeared into the cavern. I've got a hunch, though, that it's not just a cavern, but a tunnel, leading through to some underwater world. That series of sub-sea earthquakes probably opened it up; and now these devil-octopi are free to pour out. I've _got_ to find out what's what, and that's why I'm going down again as soon as the torpedo system's ready!" Keith and Robert Knapp were in the _Falcon's_ chart room. On the table before them lay a broad white map with a cross-mark indicating the position of the mysterious dark cavern. Wells was striding up and down like a caged tiger in his impatience to be off. Every other minute he glared down to where the _NX-1_ lay alongside. On her conning tower stood the tall blond-haired figure of Graham, the first officer of the day shift, supervising the final details of the work of installing a system of jury controls whereby the submarine's torpedoes could be fired from her control room.