hole, aperture, slit, cave, den
穴 centers on an opening or hollow space: a hole, slit, or aperture, and by extension a cave or den as a natural hollow.
穴 is a pictograph of a cave or hollow opening. The modern form retains the roof-like 宀 and the opening 八, suggesting a covered hole.
The top 宀 looks like a roof, and the bottom 八 looks like an opening. Together they form a roof over a hole: a cave or den.
For ケツ, imagine a kettle (ケツ) sitting in a cave: the kettle's sound cues ケツ while the cave is a hollow space.
hole; opening; orifice; perforation
ass; arse; buttocks
pore (of the skin)
pitfall; trap; pit
good place not many people know about; great little-known spot; good out-of-the-way place; well-kept secret; hidden gem of a place
filling (up) a hole
grave (pit)
large hole
conger eel (esp. the common Japanese conger, Conger myriaster); garden eel
cave; cavern; den; grotto
air hole; windhole; air vent
secret passage; underground passage
cave dwelling; troglodytism
knothole; peep-hole
pit; shaft
badger
drilling; hole punching
keyhole
anus; asshole; arsehole
cave; tunnel