The sewer is the conscience of the city. Everything there converges and confronts everything else.
Hugo takes something filthy and makes it speak. The sewer becomes a symbol for what society hides: waste, secrets, and the parts of life people pretend are not there. Calling it the city’s “conscience” suggests that truth lives in the places no one wants to see. In the novel, this imagery matters because Valjean moves through literal darkness while making moral choices. The idea that everything “converges” below ground also echoes how all classes and stories collide in one city. Readers who have lived with secrets or shame may feel the sting of recognition here. The quote says that what is buried still matters.