Minecraft wiki entry for Nether_Fortress, associated text: Nether fortresses are large complexes, composed of nether bricks materials, that are supported by massive pillars that tower high above the lava oceans. The fortress generation starts with a plain four-way crossing centered at chunk coordinates 11, ~, 11 of the designated chunk. A fortress has two areas, an exterior area of open bridges and an interior area of enclosed corridors. Both the bridges and corridors can end in a "broken" structure or may simply end without elaboration. Fortresses can tunnel through netherrack, giving the "exterior" areas an appearance of tunnels with nether brick floor and netherrack walls and ceilings. At broken sections the terrain is not cleared, which may create a tunnel that leads straight into a wall of netherrack.