"Oona and her best friend Otto love to search for treasure...and often find trouble instead. Messy trouble. Tricky trouble. Even shark-related trouble. That's never stopped them before, though! After all, no proper treasure hunt is without some adventure. But when the grandest treasure yet is stuck in a deep, dark rift, Oona's not sure if she can dive right in. What might be waiting for her in those unknown wates?"-- Publisher's description.