Book 5 : The Mountain of Adventure by Enid Blyton

Surely a peaceful holiday in the Welsh mountains will keep the children out of trouble. But the mystery of a rumbling mountain soon has them thirsty for more adventure. Philip, Dinah, Lucy-Ann, Jack, and Kiki the parrot are determined to explore the mountain and uncover its secret, but first they must escape from a pack of ravenous wolves and a mad genius who plans to rule the world.