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hollow; cavity; hole
「洞」 has other readings with different meanings.
木に大きな洞がある。
There's a big hollow in the tree.
洞の中にリスが住んでいた。
A squirrel was living in the hollow.
古い木の洞から虫がたくさん出てきた。
Many bugs came out of the hollow of the old tree.