His echoes rolled away into nothing, and it occurred to Nash that if he wanted to live, the only way was to retrace his footsteps . . .

Four sharp tugs on his safety line . . . Natalie – no doubt worrying about where he’d got to. Reluctantly, he had backed up. Halfway out of the narrow squeeze, the slight drag on his regulator told him the two-litre mini tank was running low; its 400 litres of compressed air was only good for six minutes at this depth, but Nash loved the freedom it gave him to check out promising leads.

Back at camp, he elaborated on his findings over a strong cup of coffee.

‘I think it’ll keep going all the way to Cocklebiddy. But it’s tight, Nat. I’ll need the six-litre bale-out cylinder. Pushing it ahead of me, I reckon I can just about wriggle through there.’

‘Just how small is this squeeze?’ Natalie asked, anxiously.

‘Remember the last time you had constipation?’ He began to chuckle, when suddenly she burst into tears. He looked at her in bewilderment. ‘What’s wrong?’

‘I was scared.’ Her shoulders shook. ‘Why did you just take off and leave me there?’

‘Babe, it’s OK . . .’ He tried to take her hand.

‘No, it’s not OK. You said three or four minutes, and you were gone for nearly seven. Do you know what it’s like not knowing if you’re ever coming back?’

‘What’s got into you? I was just around the corner. It was nothing. Come on, Nat, you’re overreacting.’

She was pale and reserved the next morning.

‘Did you sleep badly?’ he asked.

‘I think I might be coming down with something.’

He let her brush off his suggestion that she stay in camp. The connection to Cocklebiddy was there – he knew it – and someone might beat him to it.

Giving Natalie a cheery thumbs up, he swapped his rebreather for the smaller six-litre tank and entered the passage. Exley’s rule of thirds meant he had six minutes of exploring before he needed to return, with a third of his gas intact for contingencies.

Retracing his route to the dog-leg he tied a new line to the existing one.

Man, did he love breaking new ground!

Squeezing after his small tank, he made his way down a gradual slope, heart beating faster as the squeeze widened out into kinder dimensions. But around the next bend, it abruptly terminated in a narrow, impassable slot.

Frustrated, Nash lay on his side and shone his light through. To his amazement, there was nothing on the other side but a dark blue void. A chamber, and a bloody big one at that! Perhaps he’d been too hasty in his judgement. Yes, the slot was narrow, but it was also a good metre wide, and barely half that long.