New Zealand’s Pearce Resurgence is legendary among expert cave divers for its depth and difficulty. When an Australian explorer became obsessed with diving deeper into it, he hatched a daring plan that could cost his life

Deep in a valley in the New Zealand wilderness, clear cold water rushes across moss-covered rocks.

In the morning the mist rises up and lays across the valley. In the afternoon sun glints on the Pearce River, shafts of light filter through native tree canopy. It is a fecund, primeval place. The water has flowed down through tunnels in Mount Arthur, in South Island’s north-west, to meet at the Pearce Resurgence at its base. On the surface it looks innocuous, a calm pond. But beneath it is one of the largest and deepest cave networks in the world; unfathomable, an unknown habitat, seemingly bottomless.

Its difficulty is legendary among cave divers. To none more so than the anaesthetist and underwater explorer Dr Richard Harris, who has plunged into its dark recesses many times before he attempted a dive that no one had done before.. “It’s an intensely intimidating place to be,” Harris says in the documentary film Deeper, released this week.

“You enter this cave that just seems to swallow you as you go into it. It’s black. You come to this abyssal drop which goes over an edge and drops down to 100 metres in depth.”