Every one thousand years finds a new modern age. Whether it be by senility or a catastrophic change, villages find themselves rebuilding on the bones of everlasting empires.
Four hundred years ago, the people of Atrua made their homes in the Underground. The sole survivors of the day the sky glowed red. Though records were lost of what exactly happened, it left the air thick with ash and radiation that still leaves the sky polluted to this day.
In the early years, these people lived separate from the harmful sun, waiting out the decades it would take for the toxic rains to wash out the billions left dead on the surface, and for the thick ash clouds to disperse. The ripped ozone and surface clouds did their best to paint the world in gray, letting the unattended Nuclear factories and power plant explosions give the world pops of red. Leaving any remaining life the hardiest Earth has seen.
It was only when the air became a little more breathable, the clouds not as thick and sun not as blistering, that green sprouted from the frozen ground and some groups of humans, tired of the darkness, left the underground to make their own lives, letting those who stayed continue their lives in the places that saved their bloodlines generations ago.
A new climate had since taken root to help bury the past, covering most of the world in a blanket of snow and ice. Trees grew and animals made footholds to survive. Though the world was healing, it was still scarred, old civilizations laying ruined in the snow.