John Hopkins

He continues, plummeting through the fathomless darkness, the pressure increasing with every meter he descends.

The light grows bigger as he nears it, but it remains unmoving.

With steely calm, Don pushes on towards the bottom of the cave, 230 meters, 240.

He is now deeper than he has ever been before.

And then, at about 250 meters, there is a sinister crack, followed by a dull thud.

To an expert like Don, it's a grimly familiar sound.

The rebreather controller on his left wrist has imploded under the immense pressure at this depth.

Things have just got a whole lot more complicated.

Don must now manually insert gas into his breathing apparatus using another controller, and it's a tricky task.

At these depths, too much oxygen becomes toxic and can cause severe convulsions, which would likely prove fatal.

Even as he tries to delicately feed oxygen into the system, the levels of the gas in his rebreather grow dangerously high.

Don is forced to switch to his open circuit backup tanks.

But that comes with another problem.

Back to plan A. Don needs to get his rebreather stabilized soon.

If he can't, he may not have enough gas in his backup tanks to reach the nearest emergency cylinders nearly 100 meters above.

Drawing on his years of training, he performs a procedure known as flushing the loop, reducing the oxygen in his rebreather to safe levels so he can switch back to the machine.

Slowly, he gets things under control.

Crisis averted, for now.

But manually inserting gas and monitoring his levels is going to be a full-time job.

All his attention now has to be directed towards his own self-preservation.