The Mountain Of adventure: the adventure Series (Book 5) By Enid Blyton

A remote mountain range, a hidden fortress, and an eerie thunder that never stops! What starts as a scenic holiday saan plunges the children into the heart of a dark mystery. The locals whisper of strange disappearances and hidden tunnels that lead deep into the earth. When Lucy-Ann hears voices echoing from inside the mountain itself, and a flock of sheep vanish without a trace, Jack, with Kiki at his side, decides to investigate. But their discoveries only raise more questions: Why are planes flying overhead at night? Who built the steel door buried in the rocks? When Dinah is lured away by a strange whistle in the trees, and doesn't return, must the others follow a hidden trail into the mountain's heart?