The Underworld

Deep under the Cavern Layer the underworld begins. The following monsters begin to spawn at around -3100ft regaurdless of background change: The Fire Imp and the Bone Serpent. Imps shoot fire through walls, big bone serpents pretty much destroy your health, alot of lava and hellstone.

This is the top tier for now, giving you access to armor, weapons and tools.

Monsters in The Underworld

 * Fire Imp
 * Bone Serpent