Pompeii

be a marked increase in the ratios S/C, SO2/CO2, S/Cl, as well as the total amount of HCl.... A marked increase in the proportions of mantle components is often a sign that magma has risen into a dormant volcano and that an eruption may be expected.'
Volcanology (second edition)

    An aqueduct was a work of Man, but it obeyed the laws of Nature. The engineers might trap a spring and divert it from its intended course, but once it had begun to flow, it ran, ineluctable, remorseless, at an average speed of two and a half miles per hour, and Attilius was powerless to prevent it polluting Misenum's water.
    He still carried one faint hope: that somehow the sulphur was confined to the Villa Hortensia; that the leak was in the pipework beneath the house; and that Ampliatus's property was merely an isolated pocket of foulness on the beautiful curve of the bay.
    That hope lasted for as long as it took him to sprint down the hill to the Piscina Mirabilis, to roust Corax from the barracks where he was playing a game of bones with Musa and Becco, to explain what had happened, and to wait impatiently while the overseer unlocked the door to the reservoir – at which moment it evaporated completely, wafted away by the same rank smell that he had detected in the pipe at the fishery.
    'Dog's breath!' Corax blew out his cheeks in disgust. 'This must have been building up for hours.'
    'Two hours.'
    'Two hours?' The overseer could not quite disguise his satisfaction. 'When you had us up in the hills on your fool's errand?'
    'And if we had been here? What difference could we have made?'
    Attilius descended a couple of the steps, the back of his hand pressed to his nose. The light was fading. Out of sight, beyond the pillars, he could hear the aqueduct disgorging into the reservoir, but with nothing like its normal percussive force. It was as he had suspected at the fishery: the pressure was dropping, fast.
    He called up to the Greek slave, Polites, who was waiting at the top of the steps, that he wanted a few things fetched – a torch, a plan of the aqueduct's main line and one of the stoppered bottles from the storeroom, which they used for taking water samples. Polites trotted off obediently and Attilius peered into the gloom, glad that the overseer could not see his expression, for a man was his face; the face the man.
    'How long have you worked on the Augusta, Corax?'
    'Twenty years.'
    'Anything like this ever happened before?'
    'Never. You've brought us all bad luck.'
    Keeping one hand on the wall, Attilius made his way cautiously down the remaining steps to the reservoir's edge. The splash of water falling from the mouth of the Augusta, together with the smell and the melancholy light of the day's last hour, made him feel as if he were descending into hell. There was even a rowing boat moored at his feet: a suitable ferry to carry him across the Styx.
    He tried to make a joke of it, to disguise the panic that was fastening hold of him. 'You can be my Charon,' he said to Corax, 'but I don't have a coin to pay you.'
    'Well, then – you are doomed to wander in hell for ever.'
    That was funny. Attilius tapped his fist against his chest, his habit when thinking, then shouted back up towards the yard, 'Polites! Get a move on!'
    'Coming, aquarius!'
    The slim outline of the slave appeared in the doorway, holding a taper and a torch. He ran down and handed them to Attilius, who touched the glowing tip to the mass of tow and pitch. It ignited with a wumph and a gust of oily heat. Their shadows danced on the concrete walls.
    Attilius stepped carefully into the boat, holding the torch aloft, then turned to collect the rolled-up plans and the glass bottle. The boat was light and shallow-bottomed, used for maintenance work in the reservoir, and when Corax climbed aboard it dipped low in the water.
    I must fight my panic, thought Attilius. I must be the master.
    'If this had happened when Exomnius was here, what would he have done?'
    'I don't know. But I tell you one thing. He knew this water better than any man alive. He would have seen this coming.'
    'Perhaps he did, and that was why he ran away.'
    'Exomnius was no coward. He didn't run anywhere.'
    'Then where is he, Corax?'
    'I've told you, pretty boy, a hundred times: I don't know.'
    The overseer leaned across, untied the rope from its mooring ring and pushed them away from the steps, then turned to sit facing Attilius and took up the oars. His face in the torchlight was swarthy, guileful,