John Hopkins

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.

is losing consciousness.

With the edges of his vision darkening, Don has just seconds before blacking out completely.

If he does, he'll drown.

Don's movements become automatic, mechanical, as his ingrained training and muscle memory take over.

Divers using rebreathers are taught to change instantly to their open-circuit backup tanks if they sense themselves starting to lose consciousness.

Don has practiced the training drill enough times and taught it to hundreds of students.

But this time it's for real.

Don's mouth closes around his scuba regulator and the gas flows into his lungs.

But then, the world begins to spin.

His vision blurs as he starts to rotate through the water.