Atiu Island. Cook Island. Polynesia. South Pacific Ocean. A guide shows the details of the impressive Kopeka Bird Caves in Atiu.  One of the longest and most spectacular caves of the four islands is to the south-east of Atiu, Anatakitaki or the Cave of the Kopekas. This is a large solution cave, a series of large chambers and corridors, with an estimated length of 1 km. A fresh-water lake can be found at the bottom of the cave. The cave itself has a collapsed entrance. Out of this entrance several collapses open to the jungle, where vegetation penetrates into the cave. An especially nice view is of the dangling lianes, the "bell-pulls". The cave is very nicely decorated with white stalactites, stalagmites and flowstones. The rich and colourful speleothems are typical phenomena of tropical cave development. As a consequence of the relatively shallow and strongly fissured rock layer above the cave, roots of various dimensions have broken through. Some of the roots have also penetrated the speleothems. The Kopeka are tiny birds, very much like swifts, which nest in huge numbers inside the cave. When they come out to hunt insects they are never seen landing. They only rest in the cave. Inside, in the darkness, they make a continuous chattering, clicking noise, which they use to find their way around. (Like bats, but the bat sounds, of course, cannot be heard by the human ear.)