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sinus; cavity; antrum
neighborhood (administrative division in North Korea)
「洞」 has other readings with different meanings.
山の奥に大きな洞がある。
There's a big cave deep in the mountain.
洞の中は暗くて涼しかった。
Inside the cave was dark and cool.
子供たちが洞の入り口で遊んでいたよ。
The kids were playing at the entrance of the cave.