Just then, from the sunken steps, there came a dull clunk. Everyone froze and Adira sucked in a breath, before roaring over their heads, “The door!”

She pushed past Matt, grabbed the book and jammed it into Abrams’s bag. The major rolled it tight, and then stuck it into yet another plastic bag. He unzipped his suit and pushed it inside. He was already starting to sprint for the steps.

“Let’s go, let’s go, let’s go.”

They all ran now, flinging priceless relics out of the way, Abrams shouldering aside an ancient wooden crucifix, dark stains on each of its arms, and then all throwing themselves into the water. Vibrations filled the chamber, and they could feel the dull thunk of heavy metal and stonework gears grinding against each other right through their skin.

Matt remembered the corridor was about twenty feet long – short, but narrow, so they could not all travel side by side – someone would be last. He tried hard to remember how long it had taken the thing to open – five seconds, ten, twenty? Please be twenty, he prayed.

Up ahead, Matt could see a light waving back and froth – Hartogg or Andy, signaling – so close. They came to the end and the huge door began its slow swing. Matt turned and saw the massive stone cogs working over each other as the clockwork machine worked to draw the gigantic slab up tight against the crevice wall.

Adira went through, and she dragged Abrams with her, keeping him close – Of course, thought Matt, he now has the prize. Matt shoved Tania forward, and she swum torpedo-like through the rapidly narrowing gap. Hartogg reached in and held the crowbar between the wall and door, waiting for it to impact with the inch-thick hardened steel. Matt kicked furiously, and, as he swam through the gap, the slab reached the crowbar and crumpled it as though it were a soda can.

Matt breathed hard, feeling dizzy from the exertion. Andy swam over and grabbed his arm, his eyes troubled behind his mask. Matt nodded in return, but the geologist pointed up, and he realized the concern wasn’t for him.

The moon was still a distorted glow high above the crevice, though it had passed its peak, and the first thing Matt noticed was all the sea life had disappeared – then he found out why. Overhead, a huge shape glided past. Then another.

Hartogg grabbed Adira, and used his hands and eyes to try and convey the trouble. She simply nodded, and from her calf drew a long diver’s blade. She held the knife up to them, pointing at each. Together the divers drew forth their knives, and followed Adira as she swam up and out from the island shelter.

At the very edge of their rocky fortress, she stopped and turned, holding up three fingers – she counted down, three, two, one, and then brought her fist down. She spun and started to propel herself fast toward the position of Mahmood’s boat.