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Webster's Dictionary (1865)

Ŭn'der-world, n.

noun

(ŭn'der-wûrld), n.

  1. The lower or inferior world. “That over-spreads this under-world.” Daniel.
  2. The inferior or subordinate portion of mankind. [Rare.] Atterbury.
  3. The portion of the world which is beneath or beyond the horizon; the opposite side of the world. [Rare.]

Fresh as the first beam glittering on a sail, That brings our friends up from the under-world. Tennyson.