W hen I was a student, my girlfriend and I went on a winter weekend trip to Paris. We wore long black coats and stayed in a forlorn hotel in a narrow street in Bastille. We spent the weekend drinking in peculiar half-empty bars, and our one “activity” was to go on a romantic tour of Paris’s sewers.

Descending a stone staircase from street level, visitors step down from the busy riverside pavement into the cool gloom where the sewer’s vapours slowly envelop you. A rush of water can be heard, a moistness in the air becomes apparent, and ahead, a wide river flows from a tunnel into a well-lit central atrium.