The Novigrad sewers are located underneath the streets of Novigrad between Hierarch Square and the Oxenfurt Gate. Geralt had to venture down there a couple of times, such as the time when he was wandering the sewers, he came across Professor Vairmont Jonne trying to open a stone sarcophagus. Geralt mistook him as a grave robber until he introduced himself. Geralt helped him open the sarcophagus and they ran into a rather irritable katakan. Apart from this Geralt found nothing else of mention in the Novigrad sewers.