‘Waterproof concrete,’ Costas said. Jack could see the cone of light from his headlamp a few metres ahead, aimed at a section of the conduit wall which had partly cracked and crumbled.
‘Another Claudius speciality,’ Jack replied, coming up behind him. ‘It’s how they built the underwater moles of his great harbour at Ostia, and what they used to line aqueducts. In here it was probably used to keep groundwater from leeching down into the conduit, contaminating the springwater. The key ingredient of hydraulic concrete was a dust called pozzolana, from ancient Pozzuola. That’s Puteoli on the Bay of Naples, beside the Phlegraean Fields.’
‘Small world,’ Costas murmured as he pressed ahead.
Jack passed the damaged section, and then came under Costas’ legs where he had stopped, about fifteen metres beyond the columns that had marked the entrance to the conduit.
‘It’s all choked up,’ Costas said. ‘It looks as if there’s been a collapse.’
‘A dead end?’ Jack said.
Costas bent over and delved into the tool pocket on his e-suit. He produced a device about the size of a spoon, activated it and held it out in front of him. Jack watched the flashing red light turn to green. ‘The water current meter shows we’ve still got flow. Wherever the spring is, it’s still ahead of us.’ Costas pocketed the meter, then looked at the gauge on his wrist. ‘And we’re still going up at a slight angle, about ten degrees. At this rate we’ll break surface about twenty metres ahead, if the tunnel continues at the same angle beyond this rubble.’
Jack edged under Costas, and peered at the jumble of tufa fragments on the floor of the tunnel. He reached down and shifted one, then moved several more. ‘Take a look at this,’ he said. ‘There’s a crack underneath us, a fissure in the base of the tunnel. It must have split open when the quake brought down the ceiling. We might be able to get through.’
Costas dropped alongside Jack, and looked into the hole, angling his head so that the beam shone deep inside. ‘You may be right,’ he said. ‘It widens ahead of us, maybe body width, and goes on as far as I can see. The rubble seems to have compacted at the top of the fissure, and not fallen into it. If we can clear the first couple of metres or so, we might reach the point where the fissure’s wide enough to fin through.’
‘My turn to take the lead,’ Jack said. Costas dropped back and peered at him closely, his visor almost touching Jack’s, and made the okay sign. The two men knew each other too well, and words were unnecessary. It was always the second sump that did it for Jack, the realization that escape was no longer straightforward, that he would need to go back through several submerged spaces before reaching the final passage to freedom. He had survived a near-death experience as a boy diving a sunken mine shaft, when his air had cut off and his buddy had saved him, and the memory rose up again every time he confronted similar circumstances, every time his mind began to lock into that sense of deja vu. He had already felt the icy grip of claustrophobia before he saw the inscription, and now he needed all his reserves to fight it, his own secret battle that only Costas knew about. Taking the lead helped him to focus, to concentrate, to see the objective ahead as his own personal quest, to feel responsibility for one who now came behind him.
‘We’re still at about six metres water depth,’ Costas said. ‘By my reckoning, we’re only about thirty metres from the point directly below the House of Augustus and that temple, where we were sitting on top of the Palatine.’
‘Okay. Here goes,’ Jack muttered. He angled down and pulled himself through the crack. He finned hard, but got nowhere. He was beginning to hyperventilate. He closed his eyes, then felt a jostle from behind. ‘Your coil of rope caught on a rock,’ Costas said. Jack felt a hard push, and then was floating free inside the fissure, which had quickly widened to about two metres. He realized that he was dropping, and dropping fast. He looked at his gauge. Fifteen metres depth already. He must have deactivated the automated buoyancy control as he squeezed into the fissure, and he fumbled with the controls on the side of his helmet. There was a hiss of gas into the suit and he slowed down, reaching neutral buoyancy at eighteen metres. For the first time he looked along the length of the fissure ahead of him. The water was still crystal clear, and he could see horizontally at least thirty metres, to a point where the rough volcanic tufa walls on either side seemed to join together again. He looked down. There was nothing, a yawning blackness, an abyss like he had never seen before, deep below the heart of one of the world’s most ancient cities.
He heard grunting and cursing through his intercom, and looked up to see Costas part-way into the fissure. He began to swim back up to help him, and then Costas was through, dropping down until they both came level at twelve metres depth. ‘This place is phenomenal.’ Costas was still panting from his exertion, but was peering down. ‘The crack of doom.’
‘I can’t see the bottom,’ Jack said. ‘It must be at least fifty metres below us, maybe more.’
‘I didn’t wager for a decompression dive under Rome,’ Costas said. ‘We haven’t got the gas for that.’ They both checked the readout inside their helmets, which showed the gas mixture from their rebreathers adjusting for depth. ‘I’d say half an hour, no more, with a twenty-five-metre maximum. Any deeper than that and it’s a bounce dive, then we’re out of here.’
‘We may be lucky,’ Jack said. ‘Look along the top of the fissure.’ He panned his headlamp beam along, and Costas followed it. They could see the glistening reflection of the water surface at their entry point, then nothing but rock for about ten metres, then another wavering patch of white, this one at least three metres long. ‘Looks like it breaks surface again,’ Jack said. ‘Let’s go up.’
They began to swim in the direction of Jack’s beam. Costas rolled on his back, peering up and down the fissure, then looking hard at the rock directly above them. ‘This fissure’s clearly a seismic cleft, tens, maybe hundreds of thousands of years old. It looks as if it’s always been filled with water, spring-fed. Then right above it there’s that tunnel built by Claudius, buckled by a more recent earthquake. You can see sections of the Roman rock-cut ceiling from the tunnel above us. My guess is, the tunnel was never intended to break into the fissure, but extended above it to that pool we’re heading towards. The tunnel must be a kind of outlet, an overflow conduit for when the water here got too high.’
‘Look at that,’ Jack exclaimed, pointing to the side of the fissure. ‘There’s a flight of four, five rock-cut steps, leading up to the pool.’
‘It looks like a wellhead,’ Costas said. ‘Maybe this was where they accessed the spring. We’re coming up almost directly under the place where those prehistoric huts were found, the House of Romulus on top of the Palatine, about sixty metres above us.’
Jack broke surface first, then cautiously walked up the steps, craning his neck round to ensure there was ample headspace. He looked back to check that Costas was behind him, then reached down and pulled his fins up behind his calves before walking up out of the water on to a flat rock surface. He was inside another tunnel, but it was spectacularly different from the one they had come through. Jack turned around, looking. To the north, about ten metres from him, the tunnel came to an end at what looked like a small chamber, slightly larger than the dimensions of the tunnel. At the other end, about the same distance away, it opened into a rocky cavern, obscured in shadows. The tunnel itself was hewn out of the living rock, about three metres wide and five metres high, with a trapezoidal cross-section like a truncated pyramid. Jack swivelled around and scanned the whole length again, then looked closely at one wall, inspecting the ancient pick-marks. This was old, far older than anything else they had seen. He looked again. It suddenly clicked. ‘My God,’ he whispered.