Beneath our feet are countless miles of cave shafts and passages. 我们脚下有着绵延不绝的洞窟和隧道

1. This is our planet's final frontier,
   这里是地球上最后的边缘地带
2. an inner world where only the most adventurous dare to go.
   一个只有最勇敢者 才敢涉足的地底世界
3. Beneath our feet are countless miles of cave shafts and passages.
   我们脚下有着绵延不绝的洞窟和隧道
4. The Cave of Swallows in Mexico,
   墨西哥境内的燕子洞
5. 400 metres to the bottom,
   深达400多米
6. deep enough to engulf the Empire State Building.
   足以吞没整座帝国大厦

来自《行星地球 第一季 第四集 Planet Earth S01E04 洞穴》第 3 行