PLANET EARTH Caves

Underwater caving is notoriously（臭名昭著地） dangerous. When the nearest exit may be hundreds of meters 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.



Cave exploration often requires you to push yourself through narrow gaps in the rock. Cavers call such places 'squeezes.' The tighter the squeeze, the greater the chance of damaging some vital life-support system. In these conditions a diver could easily become disorientated（迷失方向） and that could be fatal. The flooded caverns can play tricks on you in other ways. What seems like air, isn't. It's just another kind of water.



This is a halocline（盐跃层） - a meeting of fresh and salt water. Fresh water from the jungle flows over the heavier salt water from the sea. The saltwater layer is extremely low in oxygen making it a particularly difficult place for animals to live. Yet some have managed it, like the remiped（桨足类动物）, one of the most ancient of all living crustaceans（甲壳纲动物）.