听力文本

So far, more than 350 miles of underwater galleries in the Yucatan have been mapped.
人们迄今为止已绘制了350英里的尤卡坦半岛地下水系图
But still, nobody yet knows the true extent of this subterranean water world.
但仍旧无人知晓这个地下水世界究竟有多大
And with good reason.
以及它形成的原因
Underwater caving is notoriously dangerous.
地下水洞是出了名的危险
When the nearest exit may be hundreds of metres or more away,
最近的出口可能远在几百米之外或者更远的地方
running out of air down here would be fatal.
在这里，耗尽氧气将是致命的
To avoid getting lost, divers carry with them a spool of string.
为了避免迷路，潜水员必须随身携带一捆轴线。

It becomes their lifeline, literally.
这成了它们的救生索，绝对如此。
The string also doubles as a measuring tape, a technique that has been used here in Mexico
这根线同时也是测量尺，在墨西哥的这个地方，这种科技被用来
to chart the largest underwater cave in the world, all 1 00 miles of it.
测绘世界上最大的地下水洞，全长达100多英里。
Cave exploration often requires you to push yourself through narrow gaps in the rock.
洞穴探险需要不时挤过一些狭窄的岩石缝隙。
Cavers call such places "squeezes".
洞穴专家称之为“瓶颈”。

视频及简介

洞穴是不受阳光控制的生物栖息地之一，但这并不意味着那里就不存在野生生物。本集节目将深入洞穴内神秘而永恒的黑暗，探索山洞、洞穴和隧洞中未知的地下世界。