What is Adit?

Legal Definition
An adit (from Latin aditus, entrance) is an entrance to an underground mine which is horizontal or nearly horizontal, by which the mine can be entered, drained of water, ventilated, and minerals extracted at the lowest convenient level. Adits are also used to explore for mineral veins.
-- Wikipedia
Legal Definition
In mining law. A lateral entrance or passage into a mine; the opening by which a mine is entered or by which water and ores are carried away; a horizontal excavation in and along a lode. ElectroMagnetic M. & D. Co. v. Van Auken, 9 Colo. 204, 11 Pac. 80; Gray v. Truby, 6 Co.lo. 278.
-- Black's Law Dictionary