Don’t be hoisted by your own petard

In the closing stages of Victor Hugo’s Les Misérables the chief character, Jean Valjean, while carrying another key character seeks to evade the authorities. He does so by traveling through the sewers of Paris, while the search for him and other rebels is focused on the streets above him. In this way Valjean is able to use a critical but commonly forgotten part of the maintenance infrastructure of the city against the city itself.