Should some secrets stay buried forever? When Cassie Hunt learns that her beloved Aunt Aggie is dying, she rushes back home to be with her during her final days. For as long as Cassie can remember Aggie has spoken about the forgotten world that exists just below their feet, in the tunnels and catacombs of the mysterious Sand House. But on her deathbed, Aggie reveals a terrible secret that she's buried as deep as the tunnels beneath the old house, and when excavation work begins on the site, Cassie is the only one who can help keep the truth hidden. With the return of an old university friend, Noah Flanagan, Cassie puts into action a plan to honour Aggie's dying wish. But it seems the deeper Noah and Cassie dig, the more shocking the secrets uncovered - and danger is never far away, both above and below the ground . . .