Mel turned in the nick of time. The monster shark was bearing down on him like an undersea express train. Overcoming a moment of panic, Mel gunned his ion drive to dodge the attack.  
As Tom watched in agonized suspense, he saw the shark's jaws open and shut in a lightning snap at Mel's outstretched arm. Its razor-sharp teeth missed their target by inches!
 
Mel's gasp of relief was audible over Tom's earphones. "Let's get out of here!" he cried, arrowing away from the man-killer.
 
Suddenly Tom realized the full extent of their peril. A long, sweeping coral reef, which extended above water, lay between them and the Sea Hound. Unless they could round the reef in time, the shark had them trapped!
 
"Quick! This way!" Tom exclaimed.
 
The shark was moving at blinding speed. As if sensing the boys' plan of escape, it launched itself in a wide curving sweep to cut them off.
 
"We can't make it!" Tom gasped. "We'll have to fight!"
 
Both swimmers were armed with skin diver's knives as a precaution. The two maneuvered to meet the killer's onslaught.
 
This time its broad nightmarish head was aiming straight at Tom. He jetted off to the right, but the monster veered instantly. Its lashing tail gave Mel a stunning blow.
 
As the shark's jaws gaped for a bite, Tom zoomed underneath the man-eater and slashed its belly with his knife.
 
The shark, maddened, thrashed the water in a frenzy. Tom moved like lightning to dodge a deadly blow from its bony tail. Again and again they felt the horrifying brush of the killer's fins or armor-tough hide. By this time, Mel had revived. Repeatedly the two boys dived to jab and slash at the shark's soft underbelly.
 
Both were nearly exhausted when the monster at last went limp and floated slowly up toward the surface. Pale with shock and fright, Tom and Mel jetted back to the Sea Hound.
 
Zimby Cox was startled by their faces when they clambered aboard and ripped off their masks. "What happened to you two?"
 
Tom told him. "Good night!" Zimby cried out.
 
After resting, Tom and Mel dived in again. This time luck was with them. In less than twenty minutes they sighted a small porpoise.
 
"Think we can lure it back toward the Sea Hound?" Mel queried.
 
"We'll try," Tom replied.
 
The creature with the bottle-shaped snout was as friendly and playful as most of its fellow dolphins. Too playful, Tom concluded, after vainly trying to tease it into chasing them. Instead of following, it would "tag" Tom or Mel quickly, then swim away, evidently expecting to be chased in turn!
 
"I give up!" Mel snorted in disgust.
 
Tom grinned and bobbed to the surface. He waved his hand several times in a prearranged signal. Zimby at last spotted him and brought the Sea Hound to the scene.
 
Raising his mask, Tom called, "Let's have the net!"
 
Zimby lowered a nylon net and some pieces of fish to the two swimmers as they came alongside. With the food as bait they tried to lure the porpoise to the seacopter. But just as they thought they had it, the monster would scoot off.
 
"It's just laughing at us!" Mel fumed.
 
At last, after winning its confidence with several bits of fish, the boys succeeded in snaring the porpoise. Tom clambered onto the Sea Hound's deck and helped Zimby haul their catch aboard. "Quacking" reproachfully, it was lowered through the hatch and placed in the tank, which was then pumped full of salt water.
 
As soon as the Sea Hound arrived at Fearing, Tom phoned Chow Winkler at Enterprises and asked him to fly out to the base.
 
"Pardner, how'd you like to ride herd on this critter and gentle it down for me?" Tom asked, when he showed Chow the porpoise.
 
It had been transferred to a huge, glass-paneled t............