窟窿
kū long · HSK 7
hole
🕳️ 窟 + 窿: both share the 穴 cave radical on top, so two cave-words stack into one hole
窟窿 doubles up: 窟 means cave or hole and 窿 means cavity, both topped by the 穴 (cave) radical. Sound hook: kū long rhymes loosely with cool tunnel, and a tunnel is just a long hole.