Bats out of hell? Not quite

DEEP in the rainforests of Sarawak lies the largest limestone cave system in the world. There is no road to Gunung Mulu, but this UNESCO World Heritage Site has an air-strip. The journey by river-boat takes ages so most visitors travel by plane.

Over 250km of the caves have been explored, about 30% of the total. Deer Cave, the most impressive, has a cavern 174m high. That’s one and a half times the height of Dublin’s Millennium Spire! Caves tend to be sterile sepulchral places, but Deer Cave teems with life. Great jet-black patches cover much of the ceiling while other areas, seen through binoculars, have a whitish pock-marked surface. The black areas are gigantic concentrations of roosting bats. The pock-marks are swiftlet nests.

There are thought to be three million bats in the cavern. Nine species have been identified, each one staying in its own area to roost and breed. Bats are nocturnal, but the place is a hive of activity 24 hours a day. The swiftlets take the day shift, flying out to the rainforest in search of insects for their hungry youngsters. They navigate within the great cave by emitting loud clicking sounds and listening for their echoes from the rock faces. The bats, of course, have developed much more sophisticated sonar, but their clicks are too high pitched for us humans to hear.