Rebreathe Deep the Gathering Doom

But Jones' Megalodon showed later that he had set the oxygen levels correctly for his depth — 1.3 bars of pressure — which should have caused the system to add oxygen at a correct rate.

Still, for some reason, nearly 200 feet down, Jones became so concerned about the gases in his rebreather that he decided on a radical action — to bail out of his loop.

Hartley, after seeing Jones with his fish, was away from him again for several minutes. And it was during that time that something went so wrong that Jones became convinced he had to seal his rebreather mouthpiece and cut himself off from its recycling gases.

Why did Jones reject his own equipment? That's what the experts have struggled to understand as they examined his gear.

Bozanic says that one thing in particular about Jones' Megalodon puzzles him: Although the equipment was in working order, the unit's diluent had been shut down.

In other words, one of the two sources of oxygen on Jones' back wasn't available to him. By itself, that isn't fatal or even unusual. Sometimes, divers suppress their diluent to help maintain buoyancy control. But Bozanic wonders if there's a chance Jones' didn't know his diluent was shut off. And when his system began to give him trouble, he didn't realize that by opening the diluent, he might have restored his loop gases to safe levels.

Another clue appears, at first glance, to be more ominous but may be only an artifact: Jones' other source of oxygen, the canister that carried pure oxygen, was empty when Bozanic inspected it. He says that seems strange, since Jones' dive lasted only 27 minutes, implying that he had begun his dive with almost no oxygen in that tank. Another explanation is that Jones had plenty of oxygen during his dive, but in the three weeks that transpired between the accident and Bozanic's inspection, the tank had slowly leaked away its gas.

More convincing evidence for what actually happened is a residue found in Jones' oxygen sensor pod, which tests the quality of gases in his loop. Bozanic found a powder caked on the interior surfaces that could have been soda lime, residual cleaning solution, or even salt. The powder was swabbed and stored at the medical examiner's office but hasn't been tested.

Heinerth reported that the powder may have produced a taste in Jones' mouth or could have irritated his breathing to the point of making him cough. Although an autopsy didn't find irritation in Jones' lungs, he may have believed he was about to get a "caustic cocktail," the common diver's term for the chemical burns in the mouth and lungs that occur when water gets into the loop and soda lime leaks out of its canister.

It's a terrifying prospect, and if the taste in his system caused Jones to believe it was about to happen, he may have decided to bail out before a caustic cocktail burned him.

And when he did, it was time to go to the backup systems. In parachuting terms, Zak's primary had failed, and with the ground rapidly approaching, he needed to pull the emergency chute.

Only Zak Jones, the careful professional, had no emergency backup.

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Bozanic can see how it would happen. He says that in more than 1,200 rebreather dives, only two or three times has he needed his emergency backup, known as bailout bottles. That may explain how even the most experienced diver could, over time, become complacent about them.

In theory, the way a diver like Jones assures that he's got backup is to bring a couple of small bottles with breathable gases for different levels on his dive. If he gets into trouble near the surface, he has a bottle with the appropriate oxygen levels for that depth. And if he gets into trouble farther down, he has another bottle with gases for the deeper water.

Each bailout bottle comes with its own regulator — a device, including a mouthpiece, that allows divers to suck out the bottle's contents only as it is needed, then expel an exhalation as bubbles.

One of Jones' bailout bottles contained 77 percent oxygen and could therefore be used only at 30 feet or shallower. On his way down, Jones would have clipped it to a descent line at that depth. Farther down, the bottle would be useless.

The other bottle Jones kept with him, attached to his left side. This one would be his bailout for deeper waters.

But there were two things that made this bottle useless as well. First, it contained 38 percent oxygen and was appropriate only to 100 feet. Jones dove to nearly 200 feet, and if he'd breathed from the bottle at that depth, it would have sent him into convulsions.

But he couldn't have taken a hit off it anyway. It had no regulator, no mouthpiece, and no easy way for him to make any use of it.