This is the biggest cave shaft in the world. 这是世界上最大的深洞

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
7. This is the biggest cave shaft in the world.
   这是世界上最大的深洞
8. Yet these depths were first explored only two years before man landed on the moon.
   然而人类直到登月前两年 才第一次进入这个深渊
9. Today caves remain the least explored places on Earth.
   今天 洞穴仍是地球上 人类最少涉足的地方
10. However, human beings are seldom the first to reach these black, damp places.
    但是 人类绝非 这个阴暗潮湿地界的第一位到访者

来自《行星地球 第一季 第四集 Planet Earth S01E04 洞穴》第 7 行