The city-state of Laurentia had an extensive underground network, which contained major utility lines for water, power, sewage and drainage. These utilities were largely automated and digitally monitored as part of the Laurentian "smart city" initiative, but a vast network of walking tunnels and major drainage pipes existed to allow maintenance and security personnel access to the systems as needed. These tunnels - colloquially known as the Sewers despite their many functions - were sealed off from the public, though they show signs of having been broken into and visited for purposes besides their intended uses on many occasions.
Due to the breakdown of social order that occurred in Laurentia's final six days - and the city's subsequent absorption into the Nexus as the Plane of Valhalla, the system suffered considerable damage and its major entrances were unsealed. The sewer network can be accessed via (and connects) every District of Laurentia except Windfall Ridge. Without reliable power, the Sewers are poorly lit and characters may see only one tile into the distance. Nothing of value can be found in the Sewers, but they offer many excellent places to hide and can also provide shelter from the light of the sun to Revenants in daytime.
In addition to (and separate from) the Sewers, the Underground includes a small underground rail tunnel that can be accessed from the Parliament on King's Island. The tunnel ends in a cave-in and its intended destination is unclear, though the shrine of the Sect of Maeval, dedicated to the Elder Power Hashaa, may be found among the rubble at the end of the tunnel.
Characters will not Respawn in the Underground, nor can Phylacteries, Conduit portals, and Stepping Stones be used there. As the Underground is a part of the plane of Valhalla, members of the Adamant Kinship will receive bonuses there. Revenants with Eye of Death who investigate corpses in Laurentia will be able to track targets in the Underground and vice versa.