Argo was an inhabited planet of the "pelagic" type. This was the homeworld of a humanoid species known as the Aquans. It was also home to other native sea life, including varieties of algae and the sur-snake.
Argo was once a land planet, but its surface became almost completely covered by water, following a series of violent seismic disturbances, leaving only a single large landmass, with a few associated outcroppings. The planet remained seismically unstable.
In 2269, the USS Enterprise was on a mission to study the effects of the quakes and other phenomena the planet had on its surface. With that knowledge, they intended to save millions of lives on a Federation planet identical to Argo that was soon to be undergoing a similar seismic transformation.
After not only learning how to redirect the seismic disruptions but also applying a technique for doing just that which employed the Enterprise's weapons, some of the land that had previously been under water was returned to the surface. This land included an ancient city of the air-breathing ancestors of the Aquans. (TAS: "The Ambergris Element")