Minecraft wiki entry for Building_a_metropolis, associated text: Great Under-Tier: This should be built at the very bottom of the city. Add lots of factories, refineries, et cetera. This place is likely to be heavily polluted and can have many workers. Add large "disposal pipes" that eject waste or slag (unwanted trash like smelted stone) either out of the city or into lava pools and fires or incinerators. Build large disposal furnaces that you can burn waste (or execute players) in.