Definition: a large hole in the side of a hill, cliff or mountain, or one that is underground Level: B1
该地区拥有世界上最大的地下洞穴系统之一。
The area contains one of the largest underground ___cave systems in the world.
第二天我去参观了一个内部有蓝色湖泊的洞穴。
The next day I went to see a ___cave with a blue lake inside it.
我们探索了山侧的一个黑暗洞穴。
We explored a dark ___cave in the side of the mountain.
