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An adit - in the Saxon region (Erzgebirge) Stolln - is a mine excavation driven into a mountain from the earth's surface, generally at a slight incline. In mining, adits are used for access to underground workings, for mining deposits or for prospecting purposes. When a mine transitions to deep mining, adits gradually become less important.
https://de.wikipedia.org/wiki/Stollen_(Bergbau)
In tunnel construction, tunnels are accessible underground passages that are either horizontal or slightly inclined. The term gallery is derived from the Old High German word stollo, which means support or pillar.
https://de.wikipedia.org/wiki/Stollen_(Tunnelbau)