This is our planet's final frontier, an inner world where only the most adventurous dare to go. Beneath our feet are countless miles of cave shafts and passages. The Cave of Swallows in Mexico, 400 metres to the bottom, deep enough to engulf the Empire State Building. This is the biggest cave shaft in the world. Yet these depths were first explored only two years before men landed on the moon. Today caves remain the least explored places on earth. However, human beings are seldom the first to reach these black, damp places. Here, live some of the strangest and least-known animals on the planet.


这是我们这个星球上人类探险活动最晚涉及的地方。这是一个只有最大胆的探险者才敢进入的内部世界。在我们的脚下有长度无法计算的众多坑穴和通道。墨西哥的燕子洞，深达400米，足以把整个帝国大厦装入其中。这是地球上最大的洞穴，但直到人类登上月球的前两年才有人对它进行探索。今天，洞穴是人类在地球上探索最少的地方。而且这些黑暗潮湿的洞穴的最先光顾者很少是人类。这里生活着地球上一批最奇特、人类了解最少的动物。