A planet transformed into an ocean world by seismic upheaval, home to the undersea Aquans in The Ambergris Element.
Argo was once a world with land and sea, but seismic disturbances submerged most of it, and its inhabitants, the Aquans, adapted to live underwater. In The Ambergris Element, the Enterprise is exploring the planet when Kirk and Spock are attacked by a giant sea creature.
Kirk and Spock are found transformed into water breathers. They seek the Aquans, who suspect them of being spies, though the council member Rila speaks for them. The cure is found in a substance from a sea creature, and Kirk and Spock are restored.