Terra's Apocalypse

Years lasting: 3054-3232
War broke out among the numerous gods, and the fallout destroyed not only themselves, but endangered Earth itself. The ripples of power from their violent combat resonated into the planet, causing one of the supervolcanoes to erupt, which then caused a chain reaction around the world. Earthquakes and tsunamis occurred with little sense or warning.
Humanity, originally having created ships to take them into the stars en masse, instead repurposed these ships when it was learned the planet was unlikely to last much longer. Working hastily to install cryo-hibernation systems and many other essential functions into the ships, engineers scarcely managed to update the ships in time. As much of the human populace was ushered into the ships as capacity allowed, but it still left 97% of people behind. The ships, redubbed as Arks, left the planet in a mass exodus.
The remainder of humanity lamented their lot, and fighting and looting became rampant as order began to break down. Some people figured out that they should try to dig into the ground, or take up residence in bunkers and old silos, and these largely are the ones who survived when the volcanoes all blew. As a volcanic winter set in and ash coated the earth, civilizations formed underground, carving out tunnels or repurposing natural caverns. For a almost two centuries, humanity remained trapped within the earth, until one day someone dared to venture to the surface.