A city has in its sewers a valuable resource, says Hugo, for it has been proved that human excrement is the richest fertilizer. Man's waste of this resource is a mad prodigality. Paris, for instance, literally throws away 25 million francs a year. Not only does it neglect a precious asset, but it contributes to its unsanitary condition by poisoning the water. To perpetuate this waste, Paris has erected a spectacular structure, the sewers, a gigantic sponge, an underground city with its squares, streets, and crossroads.
Besides their physical interest, the sewers are also psychologically fascinating. Throughout history they have been the scene of many dramas; countless pursuits have taken place in them. The sewers are a mirror of human vices. The garbage they harvest bears witness to man's fallibility and speaks out against his pretensions. Broken bottles speak of drunkenness; clothes that have been worn at the opera are rotting in the mud.
Except for a dim light filtering through openings in the sewer vault, Valjean is surrounded by blackness. Nevertheless, he must plunge into this vacuum, for Marius' condition is alarming. Valjean must trust almost entirely to chance, for he has no landmark. The only clue in the sewers' layout is their slope. He knows that the sewers descend toward the Seine. He therefore chooses to proceed uphill, for he does not want to emerge near the river among the crowd.
