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It's Thursday, October the 28th, 2004. In South Africa, the undulating plains of the Kalahari Desert stretch out beneath a pale sky,

the ochre-coloured earth and scrub brush, warmed by the brilliant sunshine.

But in an isolated corner of the country's northern Cape province, this vast rolling expanse is suddenly interrupted,

punctured by an enormous sinkhole, as if a titan's fist has punched through the landscape, leaving a deep gaping wound in the ground.

The walls of the abyss plunge down towards a small duckweed-covered pond below.

At first sight, there is nothing remarkable about it.

But beneath the surface, this pool opens up into a spectacular bell-shaped chamber, a submerged natural cathedral.

This is Bushman's Hole, one of the largest freshwater caves in the world.

Nobody is entirely sure how deep it may go.

No light or sound penetrates its depths. It is an inhospitable alien place, a dead zone.

And yet today, life has come to Bushmans.

Deep below the surface, a tiny pinprick of light descends steadily through the infinite darkness.

It's a torch belonging to 50-year-old diver Dave Shaw.

He glides gracefully downwards with single-minded focus, guided by a weighted shotline.

At around 260 meters, or 850 feet, Dave reaches the silty sloping bottom of the chamber.

And then it did something that no one has ever done at those sort of depths.

...