With a terrible tearing sound the stone ripped through the air above their heads and with a roar plunged into an already ruined house. A further cloud of dust rolled out.
Mamurra was waiting at the entrance to the other mine, which was hard up against the southernmost tower of the desert wall.
'Dominus.' His face broke into a smile.
'Praefectus.' Ballista smiled back. They shook hands then kissed on the cheek, slapping each other on the back. They had grown to like each other. Mamurra knew that, as far as the Dux Ripae was concerned, his conscience was absolutely clear. Nothing that he had said or written about him was unfair or malicious. The big barbarian was a good man. You could rely on him to do the right thing.
Ballista looked with distaste at the entrance to the tunnel – the big, roughly worked beams, the uneven floor, the jagged rock walls, the precarious hang of the roof. He stepped inside. The darkness stretched away in front of him, half-lit here and there by an oil lamp in a niche. It was strangely quiet in this mine after the noise of the other one.
'How goes it?'
'Good, so far.' Mamurra leant against a beam. 'As I said we would, we have dug deep; under the wall, the external bank, and the ditch. We have taken the tunnel out to about five paces beyond the ditch. There we have dug a short crosswise listening gallery. I found some old bronze round shields in one of the temples. I have put them up against the wall and have men listening at them.'
'Did the priests object?'
'They were rather unenthusiastic. But then, there is a war on.'
Although a slave should never initiate a conversation with the free, Demetrius could not contain himself. 'You mean it works? I had always thought that it might be just a literary conceit of the ancient writers.'
Mamurra's grin grew wider. 'Yes, it is an old trick, but it works. They amplify sound well.'
'And have you heard anything?' Ballista asked.
'Oddly, no, nothing at all. I am reasonably sure that if they were tunnelling near by we would have heard their pickaxes.'
'That must be good news,' Demetrius said. 'Either there has been a cave-in and they have abandoned their mine or it has wandered far off course and they are nowhere near our wall.'
'Yes, those are two possibilities,' Mamurra looked thoughtful, 'but unfortunately there is a third.' He turned to Ballista. 'When you and Maximus told me where their tunnel started out there in the ravine, I assumed – I think that we all assumed – that its purpose was to undermine the foundations of our southernmost tower, collapse it so that no artillery from there could interfere with their siege ramp. Now I am not so sure. It may well be more dangerous than that. Maybe they intend to dig clean under our defences and let their troops come up behind our wall. If so, they are waiting for the ramp to be near completion before they excavate the last part of the tunnel so that they can attack from two places at once.'
The whole party was silent, imagining an inexhaustible flow of Sassanid warriors pouring across the siege ramp while another erupted from the ground; imagining the sheer impossibility of the task of trying to stem both at once.
Ballista patted Mamurra on the arm. 'You will hear them coming. You will catch them.'
'What then?' Demetrius volubly clutched at this comfort. 'Will you smoke them out, throw bees or scorpions into their tunnel, release a maddened bear?'
Mamurra laughed. 'Probably not. No, it will be the usual – nasty work in the dark with a short sword.'
The arrow was coming straight for his face. With a convulsive twist, Ballista jerked himself back into cover. The side of his helmet hit the crenellation, the cheek piece scraping along the rough stone. He felt a muscle pull in his back. He had no idea where the missile had gone, but it had been far too close. He exhaled noisily, trying to will his breathing back to normal. Behind him he heard a low sob.
Keeping low, on his hands and knees, Ballista scrambled to the man who had been hit. It was one of his messengers, the one from the Subura. The arrow had gone in by the collarbone. Only the feathers still stood out. The man had his hands curled round them. His eyes were uncomprehending.
'You will be all right,' said Ballista. He ordered two of his equites singulares to carry the man to a dressing station. The guardsmen looked dubious at this fool's errand but obeyed anyway.
Back behind the parapet, Ballista steadied himself. He counted to twenty then peered out. There was the Persian ramp; there was the void between the ramp and the wall. But now the gap was less than five paces wide. From underneath the screens at the front, seemingly almost close enough for the defenders to touch, earth and rubble, the occasional tree trunk, fell into the drop.
It would be today. Even if he had not seen the Sassanid troops massing at the far end of the covered walkways he would have known that it would be today. The Persians had clearly decided not to wait for the ramp to touch the wall but to use some kind of boarding bridge. The race was on. One way or another it would be decided today.
Ballista looked round. The messenger's blood was already soaking into the brickwork, a film of dust dulling the bright-red pool. Ballista nodded to those with him and, again keeping very low, crawled to the trapdoor. Maximus, Demetrius and the three remaining equites singulares clattered down the stone stairs after him.
Castricius was waiting at the entrance to his mine. With no formalities, he told them to get ready.
Ballista had been dreading this moment. It had to come. It was inevitable. He had to do it. But he did not want to. Don't think, just act. 'Let's go.'
As they walked down into the northern mine the sunlight from the entrance soon gave out. They moved quietly, just them in the darkness. None of the oil lamps in the niches was alight. Before they entered, Castricius had checked that no one had hobnails in the soles of their boots. They had left their sword belts, armour, helmets – anything metal – above ground. A careless spark could bring on their greatest fear, a premature fire.
In the pitch-darkness they moved in single file. Castricius led the way, feeling his way with his right hand on the wall. Ballista followed, gripping the back of Castricius's tunic in his fist. Then came Maximus, then Demetrius.
The floor was uneven. Ballista's boot half-turned on a loose stone. He imagined twisting his ankle, breaking his leg, being trapped down here. He fought down a surge of panic. Keep going. Don't think, just act.
The walk defied time, defied logic. They had been walking for hours. They could have walked all the way across the plain to the Persian camp.
Something changed. Ballista could sense space opening all around him. Possibly it was the quality of sound. The echo of their footfalls came back more slowly. The air smelt strange. It brought to mind different things: a stable, a butcher's shop, a warship. But the air was less close than before.
Castricius stopped. Behind him, the others stopped. Carefully, very carefully, Castricius opened his shuttered lantern just a chink. The thin beam of light barely illuminated the far side of the cavern. He held up the lantern. The roof was lost in shadows. Bringing the lantern down again, he directed the light at the timbers which held up the roof. To Ballista's eye there seemed very few of them, and those there were impossibly slender.
'There are just enough to hold the roof,' said Castricius, as if reading the mind of his commander. 'The wood is good, well-seasoned, tinder-dry. I have coated the timbers in pitch.'
'Good,' said Ballista, feeling he had to say something.
Castricius directed the light downwards. Most of the floor of the cavern was ankle-deep in straw. Around the bases of the timbers were pigskins stuffed with pig fat. 'A few cooks may have a problem, but they will burn well.'
'Good,' said Ballista in a voice that sounded strained to himself.
'And here is the heart of the matter.' Castricius shone the light behind them. To the left of the mouth of the tunnel where they had entered there were three large bronze cauldrons raised on wooden blocks, straw heaped around them. A trail of straw ran from them back up the tunnel. 'I found some bitumen for the first cauldron. The others contain oil.'