[ INNER SHELL ]
Surrounding the original surface of [MARS] is an immensely vast shell around a thousand kilometers thick, extending the radius of the once red planet's to be more like that of earth's.
At a cursory glance. its construction is almost that of an impossible megastructure, utilizing a variety of materials akin to concrete, polymer, steel, and so on.
This is made possible as the structure within is living; forming and growing not that differently from any other living being, with all creatures living within holding a symbiotic relationship with it. Upon the surface prior to the [REDSHIFT] were the [OVERSEERS], who held domain over parts of the surface and some of the subsurface (labelled sectors), while deeper [MARS] held responsibility in maintaining the rest of the Inner Shell.
However, most people prior to the [REDSHIFT] found use in the spaces formed within the entirety of the Inner Shell's depth. In order to sustain itself because of this, the Inner Shell collects power from the sun, siphons materials from space and the original Mars, and absorbs the bodies of it's late denizens within itself, harvesting what energy remains.