III

The water felt colder to Knox today, though maybe that was just the old wetsuit he was wearing, or even apprehension about what lay ahead. There was no dispute that visibility was worse, however, the sea a thin particulate soup roiled up by waves and currents. He checked constantly behind him to make sure Boris was keeping up, because the noose was already biting into his throat. That said, he wanted to set a decent pace. The only thing in his favour was that he was the better diver; he needed to make the most of that.

The deeper you dived, the greater the pressure upon your body. One effect of that pressure was to enable your bloodstream to absorb more air. In such increased volumes, nitrogen had a heady effect, much like alcohol; and the faster, deeper and longer your descent, the worse it became. Nitrogen narcosis wasn’t in itself particularly dangerous, but it made divers overconfident, impaired their judgement and motor-function, rendered them as prone to stupid mistakes as a drunk behind the wheel. And that was what Knox needed: he needed Boris making stupid mistakes.

They reached the sea-floor. Knox checked his gauge. Thirty-one metres deep already, yet he needed to go deeper still. He led Boris past the underwater tip, pointing out shards of porcelain as he went, giving credibility to his account. They followed a canyon to the pelagic shelf. With the reduced visibility, he only barely saw the orange marker buoys of the fish aggregating device. He swam towards them, continuing the descent to forty metres, then fifty, feeling a mild buzz as the narcosis kicked in. You never grew immune to it, however much you dived. You simply learned to manage it.

Dark shadows ahead. He swam towards them. Boris gave a tug on the fishing line, constricting Knox’s windpipe, making it hard for him to breathe. He stopped and turned so that the line slackened. Boris was treading water some five metres away, holding up his gauges, his expression making it plain he wanted to know what the hell was going on, where the porcelain was. Knox pointed ahead. Boris shook his head. Knox checked his gauges. Fifty-five metres deep, just fifteen minutes of air left. No wonder Boris was getting nervous. If they didn’t start their ascent soon, they’d be at severe risk of decompression sickness. It was now or never. But how the hell to get close enough to Boris to—

The bull shark surged up from the turbid depths, its maw already open, murderous rows of white stalagmites and stalactites within, rolling up its mean black eyes to show its whites. Knox lifted his feet, thumped its snout with his flippers. It turned and swam away. He glanced around at Boris, watching it in rapt horror, giving Knox his chance. He propelled himself across, grabbed for the knife with his cuffed hands, but Boris was too quick for him, he turned and slashed at Knox’s face. Knox ducked beneath the blade, reached for Boris’s buoyancy-control device, inflated it to its maximum, then stripped him of his weight-belt.

At once Boris began rising like a runaway hot-air balloon. The fishing line quickly played out and pulled the noose tight around Knox’s throat, cutting into his windpipe, dragging him upwards with him so fast that it put them both at risk from the bends. The moment Boris reached the surface, Knox slammed on the brakes, coming to a halt at just four metres, deep enough for him to decompress safely, not that that would be much use if he couldn’t breathe.

Above him, Boris began thrashing wildly. Symptoms of the bends typically took an hour or so to present, sometimes even a day or more. Only in the very worst cases was onset immediate. His struggles pulled the makeshift garrotte ever tighter around Knox’s throat, so that he couldn’t breathe at all, his lungs screaming for air. He needed to get rid of the fishing line now, but he couldn’t even get a fingernail beneath it. He looked upwards, almost in prayer, at the very moment that a spasm of pain wracked Boris so severely that he let go of his diving knife. And Knox watched spellbound as it began fluttering down through the water towards him like a silvery leaf in an autumn breeze.