In most large metropolitan areas, the number of residents living underground is often higher than the number living above. These areas are often referred to as the warrens.
Warrens often have a wide variety of people, living conditions, and social strata. The areas closest to the surface usually hold the most affluent residents, though very few residents of the warrens would ever be considered wealthy. The warrens are known for having high rates of poverty, disease, and crime no matter how far down, though it's commonly accepted that the lowest levels are generally abject squalor.
Most warrens come with strong class divides, further reinforced by large organized crime elements. These crime organizations and families generally control multiple floors of a building's sub-basement, and usually charge exorbitant fees to anyone wishing to travel between floors.
Many warrens have to deal with infestations from vermin, dangerous subterranean creatures, and sometimes goblinoids and kobolds. Some warrens have even been overtaken by cults and religions of various forms, many of which worship surface dwellers as demi-gods, or revile them as demons.
Because most warrens extend far below the surface, they usually intersect with large sewer systems. This has led to extensive black markets and smuggling rings in many areas, and patrols through sewers by city watchmen are common in many places where smuggling is an everyday occurrance.