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cavity; hollow; void
hollow (voice, smile, etc.); blank (eyes, look, etc.); vacant (expression, stare, etc.); empty (words, heart, etc.)
「洞」 has other readings with different meanings.
枯れた大木の幹に大きな洞ができていた。
A large hollow had formed in the trunk of the dead tree.
フクロウは古い木の洞に巣を作ることが多い。
Owls often build nests in hollows of old trees.
洞のある岩場に小さな生き物が住んでいた。
Small creatures lived in the cavities of the rocky area.