adit

n. entrance (especially horizontal passage into mine)

An adit is a type of entrance to an underground mine which is horizontal or nearly horizontal. Adits are usually built into the side of a hill or mountain, and often occur when a measure of coal or an ore body is located inside the mountain but above the adjacent valley floor or coastal plain. The use of adits is generally called drift mining.
See more at Wikipedia.org...

n. adit

(n.)
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.
  

(n.)
Admission; approach; access.
  

Noun
1. a nearly horizontal passage from the surface into a mine
(hypernym) passage
(part-holonym) mine