Possible Definitions:
1. Underworld (n.) The lower of inferior world; the world which is under the heavens; the earth.
2. Underworld (n.) The mythological place of departed souls; Hades.
3. Underworld (n.) The portion of the world which is below the horizon; the opposite side of the world; the antipodes.
4. Underworld (n.) The inferior part of mankind.