In addition to the Underground tubes, London is crisscrossed with an endless array of other subterranean tunnels. From underground Roman sites, to Victorian-era sewers and pedestrian tunnels, to secret mail tunnels, there exists any number of appropriate and visually interesting settings.
Though long dry, these subterranean thoroughfares are literally everywhere beneath the city of London. The survivors of the Invasion have been exploring them in hopes of discovering hidden caches or others that have fallen beneath the notice of the demons. It seems that the forces of darkness have recently begun to take an interest in these locales as well, making for some unexpected and unfortunate encounters.