It no longer seemed possible that there could be any place but this nightmare maze of caverns creeping tortuously end on end through such an alien environment. He had lost all sense of direction. "Bearings" was only a word from a dictionary. His compass was made useless by an abundance of iron ore in the rock. He felt so disoriented and removed from the surface world above that he wondered if he had finally crossed the threshold into lunacy. The only breath of sanity was fueled by the stupendous sights revealed by the light from his lamp.

    He forced himself to regain control by playing mind games. He tried to memorize details of each new cavern and gallery, of each bend and turn of the river, so he could describe them to others after he escaped to sunlight. But there were so many of them his numbed mind found it impossible to retain more than a few vivid images. Not only that, he found he had to concentrate on keeping the Windbag afloat. Another float cell was hissing its buoyancy away through a puncture.

    How far have I come? he wondered dully. How much farther to the end? His fogged mind was wandering. He had to get a grip on himself. He was beyond hunger, no thoughts of thick steaks or prime rib with a bottle of beer flooded through his mind. His battered and spent body had given far more than he expected from it.

    The shrunken hull of the Hovercraft struck the cavern's roof which arched downward into the water. The craft revolved in circles, bumping against the rock until it worked off to one side of the mainstream of the river and gently grounded on a shoal. Pitt lay in the pool that half-filled the interior, his legs dangling over the sides, too played out to don the last air tank, deflate the craft, and convey it through the flooded gallery ahead.

    He couldn't pass out. Not now. He had too far to go. He took several deep breaths and drank a small amount of water. He groped for the thermos, untied it from a hook and finished the last of the coffee. The caffeine helped revive him a bit. He flipped the thermos into the river and watched it float against the rock, too buoyant to drift through to the other side.

    The lamp was so weak it barely threw a beam. He switched it off to save what little juice was left in the batteries, lay back, and stared into the suffocating blackness.

    Nothing hurt anymore. His nerve endings had shut down and his body was numb. He must have been almost two pints low on blood, he figured. He hated to face the thought of failure. For a few minutes he refused to believe he couldn't make it back to the world above. The faithful Wallowing Windbag had taken him this far, but if it lost one more float cell he would have to abandon it and carry on alone. He began concentrating his waning energies on the effort that still lay ahead.

    Something jogged his memory. He smelled something. What was it they said about smells? They can trigger past events in your mind. He breathed in deeply, trying not to let the scent get away before he could recall why it was so familiar. He licked his lips and recognized a taste that hadn't been there before. Salt. And then it washed over him.

    The smell of the sea.

    He had finally reached the end of the subterranean river system that climaxed in the Gulf.

    Pitt popped open his eyes and raised his hand until it almost touched the tip of his nose. He couldn't distinguish detail, but there was a vague shadow that shouldn't have been there in the eternal dark of his subterranean world. He stared down into the water and detected a murky reflection. Light was seeping in from the passage ahead.

    The discovery that daylight was within reach raised immensely his hopes of surviving.

    He climbed out of the Wallowing Windbag and considered the two worst hazards he now faced-- length of dive to the surface and decompression. He checked the pressure gauge that ran. from the manifold of the air tank. Eight hundred fifty pounds per square inch. Enough air for a run of maybe 300 meters (984 feet), providing he stayed calm, breathed easily, and didn't exert himself. If surface air was much beyond that, he wouldn't have to worry about the other problem, decompression. He'd drown long before acquiring the notorious bends.

    Periodic checks of his depth gauge during his long journey had told him the pressure inside most of the airfilled caverns ran only slightly higher than the outside atmospheric pressure. A concern but not a great fear. And he had seldom exceeded 30 meters of depth when diving under a flooded overhang that divided two open galleries. If faced with the same situation, he would have to be careful to make a controlled 18-meter (60-foot) per-minute ascent to avoid decompression sickness.

    Whatever the obstacles, he could neither go back nor stay where he was. He had to go on. There was no other decision to make. This would be the final test of what little strength and resolve was still left in him.

    He wasn't dead yet. Not until he breathed the last tiny bit of oxygen in his air tank. And then he would go on until his lungs burst.

    He checked to see that the manifold valves were open and the low-pressure hose was connected to his buoyancy compensator. Next, he strapped on his tank and buckled the quick-release snaps. A quick breath to be sure his regulator was functioning properly and he was ready.

    Without his lost dive mask, his vision would be blurred, but all he had to do was swim toward the light. He clamped his teeth on the mouthpiece of his breathing regulator, gathered his nerve, and counted to three.

    It was time to go, and he dove into the river for the last time.

    As he gently kicked his bare feet he'd have given his soul for his lost fins. Down, down the overhang sloped ahead of him. He passed thirty meters, then forty. He began to worry after he passed fifty meters. When diving on compressed air, there is an invisible barrier between sixty and eighty meters. Beyond that a diver begins to feel like a drunk and loses control of his mental faculties.

    His air tank made an unearthly screeching sound as it scraped against the rock above him. Because he had dropped his weight belt after his near-death experience over the great waterfall, and because of the neoprene in his shredded wet suit, he was diving with positive buoyancy. He doubled over and dove deeper to avoid the contact.

    Pitt thought the plunging rock would never end. His depth gauge read 75 meters (246 feet) before the current carried him beneath and around the tip of the overhang. Now the upward slope was gradual. Not the ideal situation. He'd have preferred a direct ascent to the surface to cut the distance and save his dwindling air supply.

    The light grew steadily brighter until he could read the numbers of his dive watch without the aid of the dying beam from the lamp. The hands on the orange dial read ten minutes after five o'clock. Was it early morning or afternoon? How long since he dove into the river? He couldn't remember if it was ten minutes or fifty. His mind sluggishly puzzled over the answers.

    The clear, transparent emerald green of the river water turned more blue and opaque. The current was fading and his ascent slowed. There was a distant shimmer above him. At last the surface itself appeared.

    He was in the Gulf. He had exited the river passage and was swimming in the Sea of Cortez. Pitt looked up and saw a shadow looming far in the distance. One final check of his air pressure gauge. The needle quivered on zero. His air was almost gone.

    Rather than suck in a huge gulp, he used what little was left to partially inflate his buoyancy compensator so it would gently lift him to the surface if he blacked out from lack of oxygen.

    One last inhalation that barely puffed out his lungs and he relaxed, exhaling small breaths to compensate for the declining pressure as he rose from the depths. The hiss of his air bubbles leaving the regulator diminished as his lungs ran dry.