ed 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"