Source : Webster's Revised Unabridged Dictionary (1913)

Underground \Un"der*ground`\, n.
   The place or space beneath the surface of the ground;
   subterranean space.

         A spirit raised from depth of underground. --Shak.

Underground \Un"der*ground`\, a.
   1. Being below the surface of the ground; as, an underground
      story or apartment.

   2. Done or occurring out of sight; secret. [Colloq.]

   {Underground railroad} or {railway}. See under {Railroad}.

Underground \Un"der*ground`\, adv.
   Beneath the surface of the earth.

Source : WordNet®

underground
     adv 1: in or into hiding or secret operation; "the organization was
            driven underground"
     2: beneath the surface of the earth; "water flowing
        underground"

underground
     adj 1: under the level of the ground; "belowground storage areas";
            "underground caverns" [syn: {belowground}]
     2: conducted with or marked by hidden aims or methods;
        "clandestine intelligence operations"; "cloak-and-dagger
        activities behind enemy lines"; "hole-and-corner
        intrigue"; "secret missions"; "a secret agent"; "secret
        sales of arms"; "surreptitious mobilization of troops";
        "an undercover investigation"; "underground resistance"
        [syn: {clandestine}, {cloak-and-dagger}, {hole-and-corner(a)},
         {hugger-mugger}, {hush-hush}, {on the quiet(p)}, {secret},
         {surreptitious}, {undercover}]
     3: used of independent armed resistance forces; "guerrilla
        warfare"; "partisan forces" [syn: {guerrilla(a)}, {guerilla(a)},
         {irregular}]
     n 1: a secret group organized to overthrow a government or
          occupation force [syn: {resistance}]
     2: electric underground railway [syn: {metro}, {subway}, {tube}]