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.