also: かんぽ
They survey the galleries inside the mine.
坑道 is the general term for a mine tunnel or gallery. 間歩 is a more specific, traditional term often used for older or smaller mining passages.
Standard kanji form for the reading まぶ, used in mining contexts.
Alternate reading かんぽ, less common; Restricted this reading to the 間歩 spelling.
Variant kanji form, rarely used.
Variant kanji form, rarely used.
The derivation is uncertain. The kanji 間 (interval, space) and 歩 (walk, step) may relate to the idea of a passage or step-like excavation, but this is speculative.