Five go off in a Caravan by Enid Blyton

The Five set off on a caravan holiday and befriend a boy named Ned, who lives in a circus camp nearby. When they sense something strange about one of the circus men, they begin to investigate his behavior. Their sleuthing leads them into hidden tunnels and a dangerous plot involving stolen goods. Trapped underground, they must work together to escape and bring the criminals to justice. This is a lively mix of circus color and thrilling adventure.