The Sanctaphrax Forest was a huge network of support beams and poles that kept the New Sanctaphrax rock from touching the ground after it succumbed to stone sickness. It was designed by Vox Verlix, an architect whose brilliance was only surpassed by his wickedness.
Because New Sanctaphrax was constantly losing buoyancy, the Sanctaphrax Forest perpetually required more timber. Adding to the Sanctaphrax Forest was such an undesirable job that Vox Verlix brought in goblin mercenaries to enslave the citizens of Undertown and force them to work on the project. The reinforcement of slavery and the violent action coined the Week of Blood. General Tytugg, the leader of the goblins rebelled against Vox's command and used his army of goblins to assume control over Undertown, while keeping Vox prisoner in the Palace of Statues.
Centuries later, the network of scaffolding at the surface of the phraxmines was referred to as the "Sanctaphrax Forest" in reference to the ancient structure.