Adit

An adit is a type of entrance to an underground mine in which the entrance shaft 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.

Adits have many advantages over conventional mining pits with vertical access shafts. Less energy is required to transport miners, horses and heavy equipment into and out of the mine. It is also much easier to transport coal out of the mine. Horizontal travel by means of narrow-gauge tramway or cable car is also much safer and can move more people and coal than vertical elevators. In some situations, mines with adits can be drained of water by gravity alone or power-assisted gravity.

(this space intended for disadvantages of adits - does anybody know of any disadvantages of adits compared to vertical shafts?)