His guess proved correct when the unmistakable outline of a jetmarine loomed into view. Tom flicked on the search beam for a moment, and Bud could be seen waving through the cabin window. Then the yellow glare went off, and Bud's jetmarine glided away to take up a scouting position ahead of the _Sea Hound_. An hour went by, then another. Suddenly a flash of light stabbed through the murk from dead ahead. "It's a signal from Bud!" Zimby exclaimed. Tom nodded grimly. "He's spotted trouble--probably an enemy sub." Silence settled over the cabin as Tom reached out to switch on the antisonar circuits. At that same instant a red light flashed on the control panel. "The Damonscope!" Tom cried out. "We may be over the Jupiter prober!" Cutting off the steering jets, Tom gave a brief flick on the reverse jets to halt the craft. Then he turned over the controls to Zimby and began stripping down to don a hydrolung suit. "Gallopin' guppies! What're you aimin' to do?" Chow exploded. [Illustration] "Go out and look for that missile," Tom said calmly. "It's what we came for." "Are you loco, boss? What about that sub Bud just spotted? Mebbe it's Mirov's bunch!" Tom refused to be dissuaded. After swallowing a space-plant pill, he armed himself with an underwater flashlight. "Think it's safe to show that light, skipper?" a crewman asked uneasily. "If the enemy spots it, I'm hoping they'll think it's coming from a school of lantern fish or sea anglers," Tom explained. He picked up a three-pronged digging fork with his other hand and went out through the air lock. Tom glided back to the spot which the _Sea Hound_ had just passed over and began digging into the silt. Presently he felt the fork strike something hard. "An obstruction!" Tom thought excitedly. He probed deeper. Bit by bit, a smoothly contoured and still-shiny metal surface became visible. "I've found it!" Tom's eyes flashed in triumph, his heart pounding. There was no doubt he had uncovered the nose cone of the missile which had re-entered the earth's atmosphere tailfirst! Meanwhile, Bud, keeping watch on the enemy submarine, had seen a shadowy figure glide from its air lock and head in Tom's direction. Bud donned a hydrolung and followed. "What's that he's carrying?" Bud wondered. Suddenly the answer came to him--a self-propelled underwater grenade! Horrified, Bud jetted forward, tackling the diver at full speed. A split second too late! The grenade went streaking straight toward Tom Swift! CHAPTER XX A LUCKY BLAST Tom's earphones caught the hiss of the approaching grenade. Instantly his eyes darted to the sonarscope on his wrist. A tiny blip of light was moving on the screen! Tom whirled about, then gunned his ion drive. He pushed out of the path of the grenade, which nevertheless grazed him as it streaked past. Seconds later, the grenade struck bottom. A shattering _bo-o-oom_ reverberated through the depths, and clouds of silt darkened the water into Stygian gloom.