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hollow; cavity; hole
「空」 has other readings with different meanings.
木の空に鳥が巣を作った。
A bird built a nest in the hollow of the tree.
この古い木には大きな空がありますね。
This old tree has a large hollow.
空になった幹の中を覗いてみたら虫がいた。
When I peeked inside the hollowed trunk, there were insects.