Once at the bottom he could walk more easily on the boards laid on the rubble and clay. The swill of dirty water seeped from the sides and gathered in puddles, trickling slightly down towards the tunnel mouth. He looked upwards quickly. How deep was he? He felt a flutter of panic. The walls towered up to a narrow strip of sky, and the movement of clouds across it made him reel.

He was sharply aware of the smell of it all around him-wet, earthy, moldy, as if nothing was ever dry and the wind never cleansed it.

He faced the dark hole ahead of him with a reluctance that startled him. He had never before felt such a crowding sense of being enclosed. He had to force himself to keep walking and try to dull his imagination.

The shadow closed over him. The winter daylight did not penetrate far. Beyond a few yards it was lit by covered gaslight. A naked flame could ignite the fumes in the air. He had heard of mine explosions and men buried forever in collapsed shafts. Could that possibly happen here? No, of course not! This was one straight tunnel, which was going to be bricked around, held with steel. Sewers did not collapse.

The noise of hammering and shoveling was ahead of him. He kept on walking, the water slopping underneath the boards. Where were the nearest rivers? Did anybody even know for certain? How much did the rivers secretly change course because of subsidence, the great engines above the ground shaking the earth, compressing it down, or rattling it loose? He was sweating and his heart was pounding in his chest.

He was still walking at exactly the same speed along the boards. The steadiness of his pace gave him an illusion of being in control, at least of himself. Dripping water seemed to be everywhere, a sheen on the walls in the gaslight. A rat appeared from nowhere, making him start. It ran along beside him for a dozen yards, then the shadows swallowed it up.

Ahead there were brighter lights, shouts, and the noise of pick blades striking with a sharp clang against rock and a dull thud against clay. He saw it, a machine like a huge drum, almost the size of the tunnel itself, the power of it thrumming as if it were the heartbeat of the earth.

There were at least twenty men laboring at one task or another, and not one of them looked up or took the slightest notice of Monk. The air was stale and cold and had a strange taste to it.

A man trundled past him with a barrow load of debris. Another rat shot out of the shadows, and then back in again. The sides of the tunnel beyond the last of the boards gleamed wet, and here and there were dribbles of water running down to the sodden earth.