The Furnace Layer

The Furnace Layer is a subterranean section beneath the school, much like the undercity. Unlike the undercity, the Furnace Layer still serves a purpose for Avon Grove city in that it supplies heat and energy for the surface. It houses the nuclear reactor which powers the city, as well as a source of molten metal which can be cooled for building material. The Furnace Maintenance crew is responsible for keeping the reactor under control. Any human-shaped figurines or dolls are strictly forbidden from entering the site, because if it is a voodoo doll, dropping it into lava may summon the Wall of Flesh and vaporize whoever the voodoo doll was made for. The Furnace Layer is also home to Tiddalik, the great Australian toad.

Urabrask The Hidden, also known as the Praetor of the furnace layer resides in the region. He does not interact with the furnace maintenance crew or other visiting citizens.

The Wall of Flesh was summoned once before, when a voodoo doll of the old mayor of Avon Grove City was dropped into lava. The mayor disappeared and the Wall of Flesh nearly devoured the entire city from the ground up. Luckily, Packy showed up just in time to defeat the wall of flesh. This released the ancient spirits of light and dark, and hallow and corruption biomes generated outside of the school. These areas were contained by building the Colosseum and portable sectors on top of them, though the corruption and hallow still persist beneath the earth. Packy retrieved the Breaker Blade, Sorcerer emblem and Pwnhammer from the remains of the Wall of Flesh, which he used to destroy 14 demon altars which blessed the world with cobalt, mithril, and adamantite ore. After the defeat of the wall of flesh and the destruction of the 14 demon altars, the Furnace Layer became an important source of these new rare ores, as well as hellstone when miners were equipped with nightmare pickaxes.