Chow pushed back his ten-gallon hat and scratched his head dubiously. "Wal, 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, d