The next morning, Nash mechanically geared up under a dark sky. A front had moved in during the early hours and dumped a rare drenching of rain. He’d had a tormented night, twisting and turning in the back of the Land Cruiser. At some point, he’d decided that he didn’t want to dive today, but the feeling was uncomfortably close to fear, and now he had to prove that it wasn’t.

Nash checked the rebreather. The size of a large backpack, the closed-circuit device had revolutionised cave diving by recycling the gases a diver breathed. Gone were the days of lugging multiple cylinders around for use at different depths. The rebreather contained bottles of oxygen and trimix – a combination of nitrogen, oxygen and helium – their percentages varied with depth by computer to offset the pernicious side effects of gas under pressure. Exhalations were run through a scrubbing agent to remove carbon dioxide. Nash also strapped on a small side-mounted trimix tank, which he would switch to when it was time to penetrate the squeeze.

In the middle of the saucer-shaped entrance pool lay the jagged mouth of the Octopus. While swimming down to it, a sense of foreboding came over him, as if a vestigial element of Natalie’s spirit waited here in anguish.

He should have come sooner. Paid his respects.

Free-falling the ten-metre-wide tube was like skydiving through an empty grain silo. Normally, he relished the feeling. Now, he found himself thinking how strangely devoid of life this place was. There were no fish, no crustaceans, just occasional finger-like curtains of bacteria on the rocks.

He reached the halocline – the boundary where fresh water floated on top of salty water – marked by a brownish line on the rocks. On a clear day, the tonality of the water shifted from gin clear to a brilliant blue, so alive and electric, as if the very molecules were charged with neon. The phenomenon was responsible for the formation of the Nullarbor caves. When seepage from rainfall hit the salt-water table, it supercharged the attack on the limestone. Thus, the caves quietly continued to grow, calving off blocks which could have been cut from a quarry.

At the bottom, Nash swam quickly down the main arm. The rectangular passage was about the size of a railway tunnel, but it seemed smaller and meaner than he remembered. The water on his exposed skin was cold. The rain had created turbidity, restricting visibility to five or six metres, despite his high-intensity dive lights.

After 100 metres, he reached the rockfall which had closed the arm. There was the stack of limestone blocks they had shifted to uncover the passage in the side wall. Just inside was the little six-litre tank he had abandoned to drag Natalie to the surface. No one had dived here since. This was unusual. Nash felt as if he was intruding on a grave. Or a crime scene.