Let's hear some ideas, lads.
>Earth becomes a poor slum while the wealthy elite lives underground (nicknamed the 'Mole Men') to profit and live off the meticulous and brutal physical labour the surface men provide for them. There's a revolt and the surface men charge down to the depths where they think the elite live and it turns out the elite have been dead for decades, possibly due to mutiny or famine. Cause is ultimately unknown since nobody can remember the last time the Mole Men were even spotted on the surface. The surfacers worked for themselves and lived from what they were creating yet they still felt oppressed by a non-existent entity they sought to overthrow. With this revelation the surface men start warring with each other over the last supplies and fuel they've been producing instead of actually continuing to work together to survive.