While above the overseas resort spot, the (green) Chaos Emerald starts reacting, with the fourth (blue) Chaos Emerald appearing to be under the sea. However, Sonic can't swim. That's when the group goes to the diving shop, wears the necessary equipment, and dives into the ocean. The group goes on a search while relying on the Chaos Emerald's signal, and discovers a sunken ship from the age of exploration. When they find the Chaos Emerald there, they are suddenly attacked by Eggman's robot and get in a dire situation. (thetvdb.com)