Underworld came to be when Codex of Ultimate Wisdom removed from the depths of the Stygian Abyss. The Resulting action opened vast cavernous Underworld beneath Britannia. It quickly became a breeding ground for monster which evoled into far horrendous forms than their originals were. Lord British wanted to explore this place and was captured by the Shadowlords in there. They imprisoned him in the Dungeon Doom which was located at center of the Underworld. The three Shards of Gem of Immortality were hidden there as well. Mystic Armor were hidden in lava where Stygian Abyss was. Captain Johne was trapped in the Underworld as well with his ship. There were three ways to enter the Underworld. The First was from the waterfall near Spiritwood, another was from a random whirpool on the sea and the most dangerous was through the bottom of any of the dungeons in Britannia. The only way to exit Underworld was from the eight dungeons because their lowest levels led in and out from the Underworld. When Lord British escaped from the mirror world of the Soul Stealer prison beneath the Dungeon Doom, it caused a tectonic upheveal which resulted in series of cataclysmic earthquakes which caused the collapse of the cavernous Underworld.