1.
mine tunnel; gallery; underground passage in a mine
A horizontal or inclined passage dug underground for mining or military purposes.
坑道に入る。
To enter the mine tunnel.
廃鉱の坑道が観光地として整備された。
The tunnels of an abandoned mine were developed as a tourist attraction.
坑道の奥は暗くて湿気が多く、ヘルメットなしでは危険だ。
Deep inside the mine tunnel it is dark and humid, and it's dangerous without a helmet.
Note the homophone distinction: 坑道 (mine tunnel) vs 行動 (action) vs 講堂 (lecture hall/auditorium). The kanji 坑 specifically relates to mines and excavation. Common collocations: 坑道を掘る (to dig a mine tunnel), 坑道内 (inside the tunnel). In military usage, refers to underground passages or siege tunnels. Related: 炭鉱 (coal mine), 鉱山 (mine — the site), 採掘 (mining, excavation).