Undersea civilization of Argo, a planet flooded by seismic upheaval, whose intervention left Kirk and Spock able to breathe only water.
Argo became a water world after violent seismic disturbances, and its people, the Aquans, live beneath the sea in cities on the ocean floor. They fear and distrust air breathers, whom they regard with suspicion and disgust.
In The Ambergris Element, Kirk and Spock were transformed into water breathers after a sea creature attacked their aquashuttle, and they had to find the Aquans to reverse the change. Captured and accused of spying, they were defended by the council member Rila, who argued that they deserved a chance to explain themselves.