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to cave in; to become depressed; to sink
長年の使用で椅子の座面が窪んでいた。
The seat of the chair had caved in from years of use.
地震で道路の一部が窪んでしまった。
Part of the road sank due to the earthquake.
目が窪んで見えるほど疲れていた。
I was so tired that my eyes looked sunken.