The final version is set in 2142 A.D., a century after humanity’s carelessness resulted in the collapse of Earth’s biosphere. The only survivors are a technological society in a magnificent enclosed scientifically-designed refuge city, Ecoban, and a small horde of refugees in the Wasteland just outside. The leaders of Ecoban have supported the refugees in return for their providing labor to mine the raw materials needed to keep Ecoban operational.
After a century, Ecoban’s leaders have degenerated into an indolent elite class who have reduced the “Diggers” to slavery, providing no more than the barest minimum to keep them alive. Meanwhile, the machinery sustaining Ecoban continues to pollute the world outside. The Diggers have finally grown angry enough to strike for better conditions. At the same time Ecoban’s machinery is breaking down and repairing it requires endangering the "Diggers" lives with more pollution.
(Source: Animation World Magazine)
— Description from AniList community (anilist.co).
