pit, hole, shaft
阬 centers on a hollowed-out space in the ground: a pit, hole, or shaft. The meanings all describe an excavated or naturally occurring cavity.
阬 combines the mound radical ⻖ with 亢, which likely contributes the sound. The mound radical often relates to earth or terrain, fitting the pit or hole meaning.
The left side ⻖ suggests a mound of earth, and the right side 亢 looks like a person bending over a hole. Picture someone digging a pit next to a mound.
For コウ, imagine a coal pit: coal -> コウ, and the pit is a deep hole in the ground.