The fifth book in the adventure series by the most beloved children's author in the world. Jack, Lucy-Ann, Philip, and Dinah will spend their summer vacation on a farm in Wales. On one of their outings, they lose their way and find themselves at the foot of a strange mountain. The fearless friends discover mysterious caves, a reservoir with black water, secret passages, and a peculiar radiation emanating from within the mountain. Who are these scientists who have turned the mountain's interior into a laboratory? What are the aims of their experiments? Are you ready for... adventure?