Explorer Academy: The Tiger's Nest (Book 5‪)‬

On an island nation in the Indian Ocean, the explorers venture through a vast underwater world and take part in a friendly robotics competition. But the tides of good fortune change quickly. Suddenly, Team Cousteau jumps into action to rescue a faculty member from the brink of death. Meanwhile, the team follows the Cruz's mother's clues to a magnificent tomb and center of spirituality precariously perched on the side of a cliff. There, Cruz is confronted by a familiar foe who's determined to stop him from completing his most important mission of all: Retrieving the final pieces of his mother's cipher. In this life-or-death showdown, Cruz witnesses the ultimate sacrifice and uncovers a hidden message that makes him question his own mortality.