He pulled the regulator away from his mouth and reached into the satchel that contained his reserve canister, making sure to exhale into the water as he did so. At his present depth he'd be under almost four atmospheres of pressure, and with the scant volume of air in his lungs, would put far too much squeeze on them by holding his breath.

Quickly placing the flange of its snorkel mouthpiece between his lips and gums, he twisted open the valve and breathed.

Nothing flowed from it.

Somehow he was not at all surprised.

Hang on. Small steps. One at a time.

The thing he needed to do now was to get outside the hollow. No, wait, check that. First he had to get rid of whatever encumbrances he didn't absolutely need to be carrying.

Ricci released his bulging tote and, given the extremity of his circumstances, was surprised by the keen pang of regret he felt over having to part with his unprecedented take. He almost tossed the spare oxygen tank as well, but caught himself at the last instant, pulled off its J-shaped snorkel attachment, and put it back into his satchel before letting go of the useless canister. Then he put both hands on the rocky floor of the hollow--an area he had just moments ago picked clean of urchins--and thrust backward and out through its entrance.

He tried to wring more air from his primary tank as he emerged into the eelgrass, but could scarcely get enough to fill his chest. It was like trying to inhale through a gag, or a smothering hand clapped over his mouth. Two labored inhalations later, the unit was depleted.

Ricci again felt desperation skittering around the edges of his thoughts. And again he blocked it out, like someone slamming the shutters against a cold December wind.

Exhale, he told himself. Nice and slow.

If he'd learned anything from his underwater survival training with the SEALs, it was that diving was all about balancing pressure. Internal and external, mental and physical. When you ran into trouble in the water, your immediate impulse was to focus solely on getting air into your lungs. It was what made a drowning person climb on the back of a would-be rescuer and inadvertently push him under. And it was usually a fatal error. Unless you were born with gills, you had to learn to modify your instincts. Concentrate on the balance, and the skills you'd acquired for maintaining it through controlled breathing, to maximize any available oxygen resource.

Assuming you had one.