The story is set in the future 1000 years after the “Seven Days of Fire”, a cataclysm brought on by the excessive industrialisation that mankind has undergone. It utterly destroyed industrial civilisation approximately a millennium after it began to flourish. Although humanity survived, the land surface of the Earth has become heavily polluted and the seas are poisonous. Most of the world is covered by the “Sea of Corruption”, a toxic forest of fungal plants which is steadily encroaching on the remaining open land. It is protected by large insects, including the huge Ohmu. Humanity clings on to survival in the polluted lands beyond the forest, periodically engaging in bouts of internecine fighting for the scarce resources that remain.

-- Wikipedia