John Hopkins

The darkness below offers few clues as to what is happening.

But there are no bubbles.

It's possible Dave has been delayed at some point during his descent, or whilst securing the body inside the bag.

But as Don continues down the shot line and still sees nothing, concern mounts.

He goes another 10 meters, then 20, and still there is no sign of his friend.

Don squints into the dark abyss below, looking for any indications that Dave's on his way back up.

At a depth of 150 meters, nearly 500 feet, he passes a last cluster of emergency gas cylinders that have been clipped to the shot line.

And then suddenly he sees something.

A tiny light shines in the darkness more than 100 meters below, like a single star burning in the vast blackness of the night sky.

But the light isn't moving.

As Don descends towards 220 meters, the intended rendezvous point with his friend, it remains motionless.

And still, there are no bubbles to suggest Dave is ascending.

In the gloomy depths of the cave, something has gone badly wrong.

Don has to get down there.

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.