The Sinking Island
They find a small island and run the ship ashore to make some repairs. On the island, William finds a tree marked with a charcoal X and thinks it might be from pirates or smugglars so he tries to find some tracks but unfortunately he doesn't so he asks Ronan if he can help. Ronan tells the rest that he's off hunting for treasure and helps William find it: A Gem of Brightness, standing on a pedestal. Without giving it any further thought, Ronan takes it off the pedestal and the ground begins to rumble and shake.
They run back to the ship where the rest has just finished repairs. They make it just as the island crumbles under their feet and sinks into the ocean.