She looked upwards, sucked in a deep breath, and then reached up to grab a handhold. She continued her climb, chasing the dying glow of Abby’s light. She wondered whether they would reach the top before the light went out; didn’t matter – they’d climb in the dark if need be. They’d climb until they hit a wall, and then they’d just sit down, and rest awhile. Climbing back down in the dark would be near impossible, she imagined.

She switched to auto, climbing on, not thinking, following the light, just mechanically lifting one foot, one hand after the other, grabbing the next stone or handhold, and so on. Sometimes the chute narrowed to a few feet, and they had to slither through. Sometimes she became stuck, and Jack would climb up behind her and push.

‘You need to dump the helmet,’ Jack had said when she got stuck yet again.

‘Never,’ was all she replied, and then made herself fit through the narrow gaps.

In another hour, the pipe opened up to a shelf of stone and a total blockage – it was the end of the chute, and her worst fear. There was a wall, ribboned with cracks and fissures, and seeping water. Abby felt along it, tapping her wristwatch against the stone and listening. Jack turned left and right, looking for another way – there was none.

Cate sat, her stomach filling with bitterness. “Looks like we’ve just hit our peak.”

“There’s always another way.” Jack also searched the entire length of the wall, tapping with his compass. It sounded as solid and final as death. Eventually, he and Abby sat next to her.

“Let’s take five.” He rested with his elbows on his knees. “We’ll find another way.”

Cate snorted softly. “Of course we will.”

Jack picked up a rock, hefting it, and then turning it around in his hands to examine it closer. He picked off a chunk of matrix, turned it over, and then banged it on the ground, dislodging more of the loose outer material. “Abby, shine your light over here.”

Abby took it off her head, and handed it to him. He angled it down at the stone and began to laugh softly. He showed it to them; inside was a perfectly preserved tooth, as big as his hand.

“Ugh, get rid of it.” Abby pushed his hand away.

“Carcharodon Megalodon.” Cate stared. “Seems these guys have been here for a long time. Maybe they swam in twenty million years ago, like everything else, when it was somehow open to the ocean, and then some sort of rockslide or earthquake sealed this part of the sea off, covered it over or sunk it.” She sighed. “There was so much I wanted to see here, to study.”