Matt nudged Adira and pointed up at the moon – it was like a giant floating spaceship above them, and was so bright that the divers cast shadows. Everything was a twilight silver, and small plankton, invisible before, now phosphoresced like a glowing snowstorm around them. Fish seemed oddly excited and darted in and out of the cracks in the rock, and even the lobster and huge crabs were drawn onto rock ledges to peer at the divers, like theatregoers leaning out of their boxes.

Adira nodded in return – the moon was peaking, and they were about to enter the apex period – it would be like this for another ten minutes only.

Matt noticed a few of the divers had stopped using their lights, and it gave him an idea. He tapped the rock wall to get everyone’s attention, pointed to his light and shut it off. After a few seconds everyone understood, and one after the other the lights went out.

As Matt expected, the lights weren’t needed, and were in fact hiding more than they were revealing. Back along the rift in the island, the slab of stone they had seen in the earlier dive now gave off a faint glow. He shot towards it, and saw that there was a huge symbol showing on its surface.

He approached it, Andy at his side. The geologist went to dig his crowbar in beside it, but Matt waved him away. He ran a hand over the whorls and long strokes – there was no indentation, the symbol itself gave off luminosity like a projection.

As Matt’s hand finished his tracing of the symbol, the huge stone swung as if on a hinge. They all could hear and feel the vibrations running through the water. Ancient coral and other debris floated away, but just inside, Matt could see huge cogs of a clockwork mechanism working. He’d seen something like it before – the Greek Antikythera mechanism. That was an ancient mechanical device, a computer, created over two thousand years earlier, and designed to predict astronomical positions and eclipses.

No technological device coming even close to it in complexity would exist for another fourteen hundred years. Who else but the Greeks could construct a hiding place such as this, and then use the world’s first computer to seal it away? Matt thought.

There was a flicker of shadow, as if something above momentarily eclipsed the moon’s glow, but when they looked up there was nothing.

Matt turned and gave the group a thumbs up, and went to enter, before Adira pulled him roughly back. She held a finger up in front of his face, and then turned and swam in first. In exactly one minute she returned, pointed to Matt first, then Abrams, and Tania. She held her hand up flat to Andy and Hartogg – Stay, it said.

Andy began to protest, but Adira pointed to both him and Hartogg and then the huge door and then made an opening motion with her hands and arms. Hartogg nodded, then Andy: Keep the door open.