4/5. Borneo's Deer Cave is a daytime retreat for five million bats, whose droppings support an entire community of creatures. Caverns in other countries harbour swiftlets who build nests from just saliva, and troglodyte animals that never see the light of day, such as the Texas cave salamander and Thailand's cave angel. David Attenborough narrates.
(Repeated next Saturday on BBC2) [web address removed]
Alison Graham's column: page 32; What PD James is watching: page 32; Letters: page