PLANET EARTH Caves

The Maya understood the importance of the cenotes, but they could never have known that these flooded passageways were actually the beginning of subterranean rivers, all of which eventually flow out to the sea. Salt water, unlike fresh water, does not erode limestone, so most sea caves are created by the mechanical pounding（拍打） of the waves.



The rocky outcrops of New Zealand's Poor Knight Islands are riddled with sea caves and just like those in Borneo they have become important shelters for many species. After a day feeding in the open water vast shoals of demoiselle（少女，源自法语） fish return to the caves, which they use as a refuge from predators.



For these fish the caves are a night time retreat, but they're not the only commuters in here. There are other fish working to a different schedule. The bigeyes（大眼鲷） are the equivalent of bats. Night feeders leave the cave each evening. And like all cave commuters they are most vulnerable at the scheduled time of departure. A bottleneck funnels（瓶颈漏斗） these exiting bats into dense concentrations attracting the attention of others.