The Thea Sisters and the Mystery at Sea (Bk. 6)

During a hot-air balloon flight, the Thea Sisters discover that the turtles have vanished from Turtle Island! If that wasn't alarming enough, dangerous drums of a strange oil are spotted off the shore of the island - a disaster waiting to happen! The Thea Sisters are surprised that none other than Vissa De Vissen wants to help them save the island and the turtles! All seems lost until Nicky and Paulina spot a couple of turtles entering a mysterious underground cave! There are mysteries within mysteries, but the Thea Sisters are determined to solve them all and save the day.