The Underworld was a realm that was the home of (and ruled by) the Morlocks. The Underworld of Mystic Force was not the same as the Shadow World from Lightspeed Rescue. Twenty years ago, the Master of the Underworld rose from the depths of the earth, in order to conquer dimensions. In a climactic war between the Morlocks and the Mystics, Leanbow sealed them away away once more.
Twenty years later, Koragg, The Knightwolf used Clare's Gatekeeper powers to raise the Underworld and unleashed Morticon. After his defeat, the Underworld sank away. At the end of Power Rangers Mystic Force, Udonna claimed that the Underworld did not exist after Sculpin's defeat at the hands of Itassis.
The Underworld was a dark, dense, and never-ending cave-like structure with countless stalactites and stalagmites along with ledges. Also, numerous depressions and downward spiraling tunnels were frequently occurring. At the very bottom of the largest tunnel sat the lair of The Master, where Necrolai, Koragg, (formerly) Morticon, and Imperious resided.
At the center of the throne room was a white pool that turned red to signal The Master's presence. The lair also hosted numerous passageways and corridors including lower levels. The lowest of all the levels was the Inner Sanctum where the Ten Terrors resided.