pit, hole
坑 centers on a hollow opening in the ground: a pit or hole, often man-made or natural.
坑 combines 土 (earth) with 亢, which likely contributes the sound and may suggest a hollow or raised shape, together giving the idea of a pit in the ground.
The left side is earth 土, and the right side 亢 looks like a lid over a hollow space. Picture digging into the earth and uncovering a deep pit.
For コウ, imagine a coal pit: coal -> コウ, and the earth 土 is dug out to make the hole.
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tunnel; (mining) gallery; level; shaft; pit
pit (esp. of a mine)
miner
closure of a mine; abandoned mine; disused mine
The Miner (1908 novel by Natsume Sōseki)
inclined shaft
drift (mining); driftway
entrance into drift or pit
winze (mining); well (oil, gas)
burying Confucian scholars alive
within a pit or (mine) shaft
mine accident
underground mining
underground coal mine
pithead; minehead
out of the pit
mine-pit bottom
mine timbers
pilot tunnel
salt mine