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(volcanic) crater; caldera
burner (of a boiler)
「火口」 has other readings with different meanings.
火山の火口から煙が上がっているのが見えた。
I could see smoke rising from the volcano's crater.
火口付近は危険なので立ち入り禁止である。
The area near the crater is off-limits because it's dangerous.
ヘリコプターで火口を上から見学できるよ。
You can view the crater from above by helicopter.