The Collaborative International Dictionary of English v.0.48:

Underworld \Un"der*world`\, n.
   1. The lower of inferior world; the world which is under the
      heavens; the earth.
      [1913 Webster]

            That overspreads (with such a reverence)
            This underworld.                      --Daniel.
      [1913 Webster]

   2. The mythological place of departed souls; Hades.
      [1913 Webster]

   3. The portion of the world which is below the horizon; the
      opposite side of the world; the antipodes. [R.]
      [1913 Webster]

            Fresh as the first beam glittering on a sail,
            That brings our friends up from the underworld.
                                                  --Tennyson.
      [1913 Webster]

   4. The inferior part of mankind. [R.] --Atterbury.
      [1913 Webster]

WordNet (r) 3.0 (2006):

underworld
    n 1: the criminal class
    2: (religion) the world of the dead; "No one goes to Hades with
       all his immense wealth"-Theognis [syn: Hell, Hades,
       infernal region, netherworld, Scheol, underworld]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "underworld":
   Abaddon, Black Hand, Cosa Nostra, Dis, Gehenna, Hades, Hell,
   Naraka, Orcus, Pandemonium, Sheol, Tophet, abyss, avichi,
   bottomless pit, bowels, criminals, deep space, deeps, depths,
   gangdom, gangland, gaping depths, hades, hell, infernal pit,
   infernal regions, inferno, jahannan, limbo, lower world,
   nether world, netherworld, organized crime, outer space, perdition,
   place of torment, purgatory, shades below, the Mafia, the abyss,
   the bottomless pit, the grave, the mob, the pit, the rackets,
   the syndicate, the underworld, unfathomed deeps, unknown depths