The Case of the Secret Passage (Pony Investigators Book 5)

Astride their ponies, Ginger and Oreo, twelve year olds, Amy Jo and Becky solve mysteries and crimes . While housesitting at the Funderburg home, Amy Jo and Becky discover a hidden door leading to a series of underground tunnels. Strange markings on the tunnel wall, visible only with a black light, lead them further and further into the passageways where the Pony Investigators uncover a fiendish plot. Before they can warn the police, the girls are caught in a trap where there seems no possible way of escape.