So far, more than 350 miles of underwater galleries in the Yucatan have been mapped, but still nobody yet knows the true extent of this subterranean water world, and with good reason. Underwater caving is notoriously dangerous. When the nearest exit may be hundreds of metres or more away, running out of air down here would be fatal. To avoid getting lost divers carry with them a spool of string. It becomes their lifeline, literally. The string also doubles as a measuring tape - a technique that has been used here, in Mexico, to chart the largest underwater cave in the world - all 100 miles of it.


迄今为止尤卡坦半岛上人类已经探索过的水下洞穴的长度约500多公里，谁也不清楚它们的实际长度，这是完全可以理解的。水下洞穴探险素来以危险著称。水下洞穴探险时，最近的出口可能离你数百米甚至更远，所以氧气用完可能会丧命。为了防止迷路，潜水员们会使用绕在轴上的线绳。这可以说是他们的生命线。这些线绳同时也是测量工具，他们用这种方法来测量位于墨西哥的世界上最大的水下洞穴——它长达160多公里。