Beneath Guisoreux vast catacombs house the city's dead. The catacombs are ancient, their origins lost in the mists of time, but since anyone can remember they have been watched over by the cult of Morr. They are accessible from entrances in the temple and a number of shrines throughout the city, where funeral services are held. In the well tended parts priests and acolytes of Morr pace the tunnels and tend to the crypts. What is less well known, however, is the fact that the true extent of the catacomb network is largely unknown. Beyond the most used areas, tunnels wind their way for miles. The catacombs watched over by the cult of Morr comprise only a small area of a maze of interconnected passageways and chambers constructed for a multitude of purposes by a multitude of different tunnelers. The most obvious of these underground networks being of course the city's sewer system. It intersects with the older catacombs at many points, though in theory any direct connections between the city's graves and its sewer system have long been sealed up.

Nearly every house in Guisoreux has a cellar, though in practice the less privileged households of the city tend to place less value on the correct storage of classic vintages; more often than not they are used to provide cramped and cheap accommodation, often for students (in fact, it is from this common use of subterranean rooms that students have acquired their nickname to the public: 'les taupes' - 'the moles') who appreciate being able to hide their indulgences from the gaze of decent society.

Fairly common too are underground passages which run between blocks of houses, entered from cellars or semi-secret entrances (sometimes through fountains, innocuous side doors, trapdoors in alleyways and even, if you don't mind the smell, a privy). Rich merchants pay to have a private tunnel put in from their house, ostensibly to go to worship or save themselves from the hazards and exertions of open air travel; in reality, they feel the need for an emergency escape route from the mob. It is no secret that such access points to the catacombs exist which are not guarded by the cult, but everyone prefers to believe that the network of passages is less well-used and less extensive than is in fact the case. After all, no-one likes to think that the trapdoor in their cellar might be all that separates them from gangs of Grave-robbers or worse. The majority of people are familiar with only a small number of tunnels used for regular journeys; it is good sense not to divulge the secrets of the tunnels to all and sundry. Robberies and rapes frequently take place in the darkened tunnels, and a fair number of people have simply gone into one tunnel and never come out the other end. Rumours abound of pale-faced gangs of cut-throats, apparently calling themselves 'les fossoyeurs' ('the gravediggers') who work exclusively in the tunnels, digging their own secret passages to break into other, normally safe routes. But those who have looked into the many darker corners of Guisoreux's hidden labyrinth know of far more exotic and dangerous things than a few footpads lurking in burrows...
For centuries these tunnels have been used as meeting places for those who seek to escape the prying eyes of the King's authority. Clandestine affairs, secret business deals, treacherous plots, drug-crazed orgies, all have taken place in the catacombs. Thieves, students, rakes, and lovers routinely use the passages nearer the surface to pass the Louisian wall at night for their various purposes. Elsewhere, where light never challenges the dark and slithering things feast on unguarded bodies, chaos and evil make their home.