Halcyon

Halcyon is a subterranean metropolis that serves as the setting of Below. Constructed over the course of several decades, the city is home to more than 500,000 people who fled from the bombs and reaches more than 10 miles into the Earth's crust. The city is heavily divided along class lines, with the richest residing in the depths of the city and the poorest near the surface, where the radiation is most severe.

Construction
The date that the construction of Halcyon begun is unknown, although due to its immense size it was presumably at least a decade before the Great Collapse of 2055. It was one of an unknown number of similar underground cities, and was designed to house hundreds of thousands of residents for several decades. The geographic location of the city is also unknown.

Construction of the city began by drilling a chasm into the Earth's crust using a series of gigantic borers, and when these efforts concluded the chasm extended more than 10 miles below the surface. The final borer was left behind to be used as the central anchor, or branch, of the city, known as the Spire.

Layout and Structure
While it appears to be a widely-sprawling metropolis, in truth Halcyon is nothing more than a single supertall megastructure with hundreds of branches all extended from a single central structure, the Spire.

Spire
The Spire is the central anchor point for all of Halcyon, built out of the stripped-out remains of the borer that was left behind once the 10 mile deep chamber in the Earth's crust was created to build the rest of the city.