The Collaborative International Dictionary of English v.0.48:

Mine \Mine\, n. [F., fr. LL. mina. See Mine, v. i.]
   [1913 Webster]
   1. A subterranean cavity or passage; especially:
      (a) A pit or excavation in the earth, from which metallic
          ores, precious stones, coal, or other mineral
          substances are taken by digging; -- distinguished from
          the pits from which stones for architectural purposes
          are taken, and which are called quarries.
      (b) (Mil.) A cavity or tunnel made under a fortification
          or other work, for the purpose of blowing up the
          superstructure with some explosive agent.
          [1913 Webster]

   2. Any place where ore, metals, or precious stones are got by
      digging or washing the soil; as, a placer mine.
      [1913 Webster]

   3. (Fig.): A rich source of wealth or other good. --Shak.
      [1913 Webster]

   4. (Mil.) An explosive device placed concealed in a location,
      on land or at sea, where an enemy vehicle or enemy
      personnel may pass through, having a triggering mechanism
      which detects people or vehicles, and which will explode
      and kill or maim personnel or destroy or damage vehicles.
      A mine placed at sea (formerly called a torpedo, see
      torpedo[2]
      (a) ) is also called an marine mine and underwater mine
          and sometimes called a floating mine, even though it
          may be anchored to the floor of the sea and not
          actually float freely. A mine placed on land (formerly
          called a torpedo, see torpedo[3]), usually buried,
          is called a land mine.
          [PJC]

   Mine dial, a form of magnetic compass used by miners.

   Mine pig, pig iron made wholly from ore; in distinction
      from cinder pig, which is made from ore mixed with forge
      or mill cinder.

   gold mine
      (a) a mine where gold is obtained.
      (b) (Fig.) a rich source of wealth or other good; same as
          Mine 3. --Raymond.
          [1913 Webster]

WordNet (r) 3.0 (2006):

mine pig
    n 1: pig iron made entirely from ore