l, I'm keepin' a net handy to drag you out, boss, just in case."

Tom chuckled and fitted the mask over his face, then made a clean dive
into the tank. For the next ten minutes the girls and Chow watched
wide-eyed as he swam, walked around, and went through vigorous exercises
at the bottom of the tank without once coming up for air.

"Whee!" Sandy exclaimed when Tom finally climbed out. "Make me one, so I
can take up skin diving!"

"It's wonderful!" Phyl added admiringly.

Tom took off his mask. "I'm pretty pleased with it myself," he admitted,
grinning.

The girls stayed at Enterprises for lunch. Then the group, accompanied
by Doc Simpson, flew to Fearing Island so Tom could test his invention
in deep water. Boarding a small motor launch, with Doc at the helm, they
cruised out to a suitable depth and dropped anchor.

"Don't become too confident, Tom," Doc warned. "I'll drop a signal line
over the side in case of emergency."

Tom buckled on his equipment belt and adjusted the face mask. Then he
held up crossed fingers and back-flipped over the gunwale into the
water. Chow, Doc, and the girls watched his plummeting figure fade from
view.

Tom, an expert skin diver, had never before felt such a sense of ease
and freedom under water. He was moving, light and self-contained, in a
green, magical world. With no air tanks chafing his back, he felt akin
to the fishes themselves.

"Wish I'd brought a hook and line along." He chuckled, as a school of
mackerel darted past.

Now came the real test. Deeper and deeper, Tom cleaved his way downward.
Reaching bottom, he prowled about the ocean bed for a while, then
started up again. Suddenly a stab of pain shot through his chest--a
warning of nitrogen bubbles forming in his blood!

Tom swam toward the signal cord, dangling dimly in the distance. By the
time he reached it, his muscles were knotting with cramps.

"It's the bends again, all right!" Tom realized. Gritting his teeth, he
yanked hard on the line, then summoned his strength to hang on.

Doc and Chow hauled up frantically. Tom's face was contorted with pain
when they finally got him aboard and stripped off his mask.

"Oh! How awful!" Phyl gasped.

Sandy cradled Tom's head in her lap, and Phyl held his hand
sympathetically, while Doc Simpson injected a hypodermic to ease the
pain. Chow steered the launch back to shore, and Tom was rushed to the
base infirmary in an ambulance.

Here he was placed in a decompre