Down a few stone steps he was led by the bearded crew, and then pushed through a hole in what appeared to be a heavy brick wall.

He realised at once where he was.

The gurgle of running water, the odor of foul airs came up to him.

It was the great sewer that ran from the hills through the heart of the city, flushed continuously by a diverted mountain stream that swept down from above.
He was wading in cold water over a slippery bottom, tightly held by two men, the third going ahead with the lantern.

Always ahead loomed the black, opaque circle which never came nearer, never grew smaller.