(rail transport) The man-made tunnel under the English Channel joining England and France.

(idiomatic) A round piece of jewelry with a hole in the middle, which fits into a stretched earlobe.

(informal) A soldier trained to enter enclosed enemy spaces like tunnel systems to search them and eliminate and/or capture any occupants.

plural of tunnel rat

(physics) A semiconductor diode that has negative resistance in the forward direction of its operating range due to quantum effects.

A facility, comparable to a wind tunnel, in which an underwater component may be tested in a controlled flow of water. / A tunnel used to transport water.

A test facility through which air is forced in a controlled manner so as to study the effects of flow around airfoils, aircraft, motor cars etc.

A tunnel which allows workers to maintain pipes which transport steam between a central boiler and buildings to be heated or cooled, usually in a campus setting.

plural of steam tunnel

(science fiction) a passageway enabling time travel