The Eternal City

The Eternal City is exactly what its name implies; a seemingly endless collection of buildings, parks, electrical lines and roads that stretches from horizon to horizon. Its origin lies shrouded in mystery, and no comprehensive map of the city has ever been made. It is a shifting metropolis, prone to radical transformations both at the hands of its mechanical Custodians and as a result of the unseen machinery beneath its concrete surface, the only constants in its skyline being the roads and metro-lines. So far, no explanation for the origins, age, or purpose of the Eternal City has been discovered.
No organic life can be said to be native to the Eternal City. Humans originate from one of many chambers deep beneath the city, released periodically from mechanical caskets. Most of these awakened humans lack any memory of whatever previous lives they may have lived, and those who retain their memories seem to disagree on every aspect of the past. It is widely believed that all plant and animal life originated from chambers under the city as well, although no such chambers have ever been found by explorers.
Over the course of two hundred years, humanity has slowly and laboriously mapped out thirteen sectors of the City to call their own. These Sectors form the basis of the Municipalities of Humanity, a loose band of governments who administer to their respective homes and provide protection and organization to teeming masses released from underground. A domestic lifestyle has arisen among the denizens of the Sectors, one where a registered citizen needn't worry of lacking a roof over their head or a warm meal.
Exploration is a fairly commonplace job. The collective hunger for answers is still strong, even two centuries later, and the Eternal City is in no short supply of unknowns. Broadly speaking three areas of exploration exist: the Scrapyards and Concrete Wastes in between inhabitable Sectors, the mysterious sub-levels that sink deep into the foundation, and the unending Horizon beyond the walls.
In a world of unknowns, the most common ones tend to become banal. No citizen who's been awake or alive for more than a week is intimately familiar with the Custodians, sterling steel machines of varying forms and functions that routinely patrol the city. These machines are responsible for everything from the disposal of trash to the maintenance of infrastructure, and perform routine tasks every day before returning to specially-made service elevators and disappearing back into the depths. It is a commonly-held rule not to disrupt the activities of the Custodians, both for the sake of maintaining the Custodian's patterns of behaviour and to avoid possible conflict with the mechanical species. Rumours of "Enforcer-Type" Custodians armed with vicious electrical prods are the subject of tavern stories and conspiratorial whisperings, but have never been confirmed.