John Hopkins

The divers retreat to the surface, to break the news to the rest of the team.

Meanwhile, Don continues his slow, steady ascent, all the time looking back down to see if there is any movement from Dave's light, clinging to the unlikely hope that his friend might still be alive.

Eventually he reaches 150 meters, where a group of emergency tanks are clipped.

With the gas in his own cylinders now depleted after his rebreather malfunction, he's reached them at a crucial time.

It's an unrealistic shred of hope, but hard to shake.

On the surface, dive marshal Werner has set an emergency plan into motion, and tension mounts as people wait to discover which diver is coming back.

The question is soon answered.

When Don reaches 80 meters, another support diver drops level with him.

Immediately the diver sees Don's wrecked rebreather controller.

It's obvious he has gone deeper than planned, and that can only mean there was a problem.

Don goes to write something on a diving slate, a plastic, waterproof tablet divers can scribble on to communicate underwater.

It is vital that the rest of the team knows what has happened so they can adjust their plan accordingly.

The dive has turned tragic, but there is little time now to process it properly.

All attention switches to getting Don out safely and ensuring there are no other accidents.

In the dark embrace of the cave, Don focuses on keeping his oxygen levels under control and adhering to the correct decompression stops.

At around 45 meters, he reaches the roof of the enormous chamber.

Somewhere above him is the bottleneck, which will take him back into the basin above.

Don reaches for another nearby cylinder attached to the line so he can flush his breathing loop and maintain safe levels of gas within his rebreather.

But then, with no warning, Don's surroundings suddenly turn hazy and unsteady.

His eyes struggle to focus.