noun
cave entrance; tunnel
Refers to the opening of a cave or a tunnel-like passage through rock. Often used for natural cave mouths or man-made rock tunnels.
その洞門は岩山をくりぬいて作られた。
The tunnel was carved through the rocky mountain.
鍾乳洞の洞門から冷たい風が吹き出していた。
Cold air was blowing out from the cave entrance.