What a strange world unknown to most of us lies under our feet: we live above a cavernous land of waterfalls and rushing rivers, where tides ebb and flow as in the world above. If you have ever read the adventures of Allan Quatermain and the account of his voyage along the underground river to the city of Milosis, you will be able to picture the scene of Lime’s last stand. The main sewer, half as wide as the Thames, rushes by under a huge arch, fed by tributary streams: these streams have fallen in waterfalls from higher levels and have been purified in their fall, so that only in these side channels is the air foul. The main stream smells sweet and fresh with a faint tang of ozone, and everywhere in the darkness is the sound of falling and rushing water. It was just past high tide when Martins and the policeman reached the river: first the curving iron staircase, then a short passage so low they had to stoop, and then the shallow edge of the water lapped at their feet. My man shone his torch along the edge of the current and said, ‘He’s gone that way,’ for just as a deep stream when it shallows at the rim leaves an accumulation of debris, so the sewer left in the quiet water against the wall a scum of orange peel, old cigarette cartons, and the like, and in this scum Lime had left his trail as unmistakably as if he had walked in mud. My policeman shone his torch ahead with his left hand, and carried his gun in his right. He said to Martins, ‘Keep behind me, sir, the bastard may shoot.’

‘Then why the hell should you be in front?’

‘It’s my job, sir.’ The water came half-way up their legs as they walked; the policeman kept his torch pointing down and ahead at the disturbed trail at the sewer’s edge. He said, ‘The silly thing is the bastard doesn’t stand a chance. The manholes are all guarded and we’ve cordoned off the way into the Russian zone. All our chaps have to do now is to sweep inwards down the side passages from the manholes.’ He took a whistle out of his pocket and blew, and very far away, here and again there, came the notes of a reply. He said, ‘They are all down here now. The sewer police, I mean. They know this place just as I know the Tottenham Court Road. I wish my old woman could see me now,’ he said, lifting his torch for a moment to shine it ahead, and at that moment the shot came. The torch flew out of his hand and fell in the stream. He said, ‘God blast the bastard!’

‘Are you hurt?’