The Underworld is where the methane that fuels Bartertown is produced.
Underworld is located in the catacombs beneath Bartertown.
While the origins of Underworld are unknown, and the origins of Bartertown are disputed, what is known is that the pig-shit-turned-fuel operation is the brain-child of the Imp Master, better known as the more intelligent half of the duo Master-Blaster. The King of Shit and Piss has since claimed the honor of ruling Underworld.
Underworld is home to its leader, the King of Shit, as well as to the factions that provide the labor force for the endeavour: the Swineherds, The Wretched/Refugees, as well as any Criminals being forced to serve time below the market town.
Methane fuel, mostly for use in Bartertown above.