He suddenly noticed a shadow his torch could not chase away. Stepping closer, he perceived that the wall separating two alcoves was in fact two walls. Between them was a gap of some two or three feet. He held his torch closer. A narrow corridor ran all the way to the back wall. There was nothing there. No skulls, no shelves. A corridor to nowhere.

This puzzled him.

He edged down it and within moments he was face to face with a blank wall. His hand touched cold stone. Curious, he thought. Why this space? With no answer, he turned to go but as he did, the torch flame fluttered. He stopped. Held the torch close to the corner. The flame writhed in protest. That was when he noticed the long crack in the crook of the wall.

Not a wall, he smiled to himself, but a door.

In a fit of excitement, he put his shoulder into the corner and shoved. All at once, it shifted and swung smoothly inwards. He felt like a boy again, his blood prickling with the thrill of discovery.

He went inside. The walls were no longer brick-lined but chiselled out of the native rock. The flame revealed a pathway ahead, leading downwards, deeper into the hillside. He had to stoop now and hold the flame away from him to keep it from scorching his face. To his surprise though, the air was not stale. It had a freshness to it. And then he heard another sound over the crunch of his sandals.

Running water.

His heart beat faster as he drove downwards, the rushing sound getting louder with each step until all of a sudden the torchlight expanded. He took another step and his foot jarred heavily. A splash of water. He almost fell but just caught himself against the ceiling.

He paused, catching his quickening breath. The water wasn’t deep but it was piling up around his ankles. The bed underfoot was solid and smooth like pavings. Or tiles. It was a few seconds before he realized where he was. He was standing inside an aqueduct – one of the miracles of engineering that kept the city alive. The gradient was only slight, but the stream steady. He turned into it and started wading. . .

Pushing uphill, he felt his triumph surging within, not caring how far or how long it took him, hungry only to know where the tunnel would lead. If Justinian found his way in, he could find his way out.

The torch was half-burned through now, the young monk all but forgotten. His short, staccato breaths echoed off the damp walls. He was counting. A hundred steps – two, three, four hundred. Eight more and he saw what he was seeking. Another hole in the wall.