Enid Blyton 
Five Go to Billycock Hill [EPUB ebook] 
Famous Five #16

Why should aeroplanes take off from a small and secret airfield in the middle of a stormy night? This and other questions are answered by the Five who are on a camping holiday nearby – and they take part in an exciting rescue in the dangerous tunnels of Billycock Caves. The Five are on a camping holiday on Billycock Hill, and everything seems peaceful at first. But a visit to a nearby Butterfly Farm brings them into contact with some strange characters, and the Five are curious about the aeroplanes that take off from a secret airfield-but they get a real fright when they explore the long and dangerous tunnels of Billycock Caves.