Theo, Scarlett and James were all in Mr Wiggins’ class at Bluesky school in Marbleglow - the main town in the Skylands, far to the north of Horsmarlonerpool.

One Tuesday morning, Mr Wiggins had a special announcement to make - tomorrow, the whole of their year were going on a special school trip to the White Tunnels.

The class looked at each other with a mixture of excitement and nervousness. The White Tunnels were mysterious places that began in a large cave in the rocks on the outskirts of town. But nobody knew where they went, or how far they went. Nobody had ever been brave enough to explore them fully.