In the real world, huge tunnels underlie parts of Las Vegas in order to accommodate the amazing degree to which water can rapidly collect and flood an area with nothing but desert sand to hold it back. These tunnels provide drainage on the occasions that significant rainfall strikes the area, but remain unused much of the time. In addition, the busy thoroughfares of the Strip are unable to endure the treacherous traffic that sometimes fills them and still allow for things like delivery trucks, fire trucks, and ambulances with any degree of facility. Thus sections beneath the Strip serve as underground streets where the metric tons of food, alcohol and toiletries can reach the thriving miniature cities of Las Vegas Boulevard South. In the real world, the homeless street people tell of “trolls” living in the darkness of the tunnels.
The Underside contains many different types of folk and many lost peoples, mythic races and the time lost. It is impossible to put a figure on the population of the Underside as it is always changing, always shifting, and many of the peoples within it are hard to find or count. The rivalry of the tribes and their friction with each other further complicates matters and no one has truly attempted a census.