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.
 
Tom, knocked off balance, was tumbled about helplessly by the train of shock waves. As they died away, he gradually recovered his bearings and pressed the throttle control of his ion drive. It coughed and stuttered! For a moment Tom felt a surge of panic, but the jet motor smoothed into a steady purr of power.
 
"Whew!" he thought in relief. "At least I can still get around at full speed if anything else comes at me!"
 
He had clung to the flashlight and fork despite the explosion. The blast had hurled him away from the spot where the missile was buried, so Tom began trying to locate it again.
 
But he soon realized that his efforts were hopeless. He must wait until the silt which clouded the water cleared. Now Tom feared that the explosion might have reburied the nose cone.
 
Suddenly a new worry gripped him. Had the missile's precious contents been destroyed by the blast?! Slowly he began making his way back to the Sea Hound.
 
Unknown to Tom, Bud was fighting a desperate battle with his adversary barely fifty yards away. The divers grappled each other in an octopuslike duel. At such depths, their movements were impeded, as if by oil.
 
The Brungarian pulled out the knife at his belt. Bud, a skilled wrestler from high-school days, managed to twist his foe's knife arm behind his back—then applied a punishing judo hold! The Brungarian gave an audible screech of pain and dropped the knife.
 
"Now you're coming along with me!" Bud muttered. He gunned his jet, forcing himself and his adversary toward the Sea Hound.
 
Moments later, they passed the seacopter's cabin window. Reaching the air lock, Bud hammered for admission. The hatch opened quickly and his prisoner was hauled inside. Bud followed.
 
Tom greeted him with a bear hug. "Hi, Bud, you old devilfish!" Turning to the prisoner, Tom added "Who's this?"
 
"The rat who fired that grenade at you!"
 
The prisoner was wearing a frogman costume and a mask which hid the lower part of his face. The man's dark eyes glittered in hate, as Tom ordered him to remove his mask. Sullenly the prisoner obeyed.
 
Tom gasped. "Dimitri Mirov!" The name sent a shock through the Americans aboard.
 
"Wal, I'll be jing-whistled!" Chow declared, then broke into a gleeful cackle.
 
Under their scornful gaze, the Brungarian's own eyes wavered and his shoulders slumped in an attitude of defeat. "What is the use?" he muttered. "Again I have failed. My career is over now, just like my brother's."
 
Tom seized the opening. "In that case, maybe you're ready to do some talking now."
 
Mirov shrugged. "What do you wish to know?"
 
In answer to Tom's questions, Mirov admitted that his group, composed of Brungarian rebel Navy men and rocket engineers, had sabotaged the returning Jupiter probe missile, hoping to obtain its data for their own use.<............