The Collaborative International Dictionary of English v.0.48:

adit \ad"it\ ([a^]d"[i^]t), n. [L. aditus, fr. adire, aditum, to
   go to; ad + ire to go.]
   1. An entrance or passage. Specifically: The nearly
      horizontal opening by which a mine is entered, or by which
      water and ores are carried away; -- called also drift
      and tunnel.
      [1913 Webster]

   2. Admission; approach; access. [R.]
      [1913 Webster]

            Yourself and yours shall have
            Free adit.                            --Tennyson.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Tunnel \Tun"nel\ (t[u^]n"n[e^]l), n. [F. tonnelle a
   semicircular, wagon-headed vault, a tunnel net, an arbor, OF.
   also tonnel; dim. of tonne a tun; -- so named from its
   resemblance to a tun in shape. See Ton.]
   1. A vessel with a broad mouth at one end, and a pipe or tube
      at the other, for conveying liquor, fluids, etc., into
      casks, bottles, or other vessels; a funnel.
      [1913 Webster]

   2. The opening of a chimney for the passage of smoke; a flue;
      a funnel.
      [1913 Webster]

            And one great chimney, whose long tunnel thence
            The smoke forth threw.                --Spenser.
      [1913 Webster]

   3. An artificial passage or archway for conducting canals,
      roads, or railroads under elevated ground, for the
      formation of roads under rivers or canals, and the
      construction of sewers, drains, and the like.
      [1913 Webster]

   4. (Mining) A level passage driven across the measures, or at
      right angles to veins which it is desired to reach; --
      distinguished from the drift, or gangway, which is led
      along the vein when reached by the tunnel.
      [1913 Webster]

   Tunnel head (Metal.), the top of a smelting furnace where
      the materials are put in.

   Tunnel kiln, a limekiln in which coal is burned, as
      distinguished from a flame kiln, in which wood or peat
      is used.

   Tunnel net, a net with a wide mouth at one end and narrow
      at the other.

   Tunnel pit, Tunnel shaft, a pit or shaft sunk from the
      top of the ground to the level of a tunnel, for drawing up
      the earth and stones, for ventilation, lighting, and the
      like.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Tunnel \Tun"nel\, v. i.
   To make a tunnel; as, to tunnel under a river.
   [PJC]

The Collaborative International Dictionary of English v.0.48:

Tunnel \Tun"nel\, v. t. [imp. & p. p. Tunneled
   (t[u^]n"n[e^]ld) or Tunnelled; p. pr. & vb. n. Tunneling
   or Tunnelling.]
   [1913 Webster]
   1. To form into a tunnel, or funnel, or to form like a
      tunnel; as, to tunnel fibrous plants into nests. --Derham.
      [1913 Webster]

   2. To catch in a tunnel net.
      [1913 Webster]

   3. To make an opening, or a passageway, through or under; as,
      to tunnel a mountain; to tunnel a river.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tunnel
    n 1: a passageway through or under something, usually
         underground (especially one for trains or cars); "the
         tunnel reduced congestion at that intersection"
    2: a hole made by an animal, usually for shelter [syn: burrow,
       tunnel]
    v 1: move through by or as by digging; "burrow through the
         forest" [syn: burrow, tunnel]
    2: force a way through

Moby Thesaurus II by Grady Ward, 1.0:

119 Moby Thesaurus words for "tunnel":
   abri, access, adit, aisle, alley, ambulatory, antre, aperture,
   approach trench, arcade, artery, avenue, bore, bunker, burrow,
   cave, cavern, channel, cloister, colonnade, communication,
   communication trench, conduit, connection, corridor, couch,
   countermine, countersink, coupure, course, cove, covered way,
   covert, deepen, defile, delve, den, depress, dig, dig out, dike,
   ditch, dive, double sap, dredge, drill, drive, duct, dugout, earth,
   egress, entrance, entrenchment, excavate, exit, ferry, fire trench,
   flying sap, ford, form, fortified tunnel, fosse, foxhole, furrow,
   gallery, gouge, gouge out, groove, grot, grotto, grub, hole,
   ingress, inlet, interchange, intersection, junction, lair, lane,
   lodge, lower, mew, mine, moat, opening, outlet, overpass, parallel,
   pass, passage, passageway, penetrate, portico, quarry,
   railroad tunnel, run, sap, scoop, scoop out, scrabble, scrape,
   scratch, sewer, shaft, shovel, sink, slit trench, spade,
   subterrane, subway, traject, trajet, trench, trough, troughing,
   troughway, underpass, warren, way