Argo was an inhabited planet, homeworld of a humanoid species known as the Aquans. Other lifeforms included algae, fish, and the sur-snake. Plant life included sea foliage and plants reminiscent of trees on land.
Once a land planet, Argo's surface became almost completely covered by water following a series of violent seismic disturbances, leaving only a single large landmass and some outcroppings suggesting the peaks of submerged mountains. At one point, an entire portion of a continental shelf sank within minutes. At least one sea quake occurred in the 23rd century, which the Aquans considered evocative of legends about the cataclysm.
As of 2269, the planet had undergone several surveys, but still almost completely unexplored and the existence of the sur-snake or the Aquan civilization was unknown. In 2269, the USS Enterprise visited it on a mission to study the effects of the quakes and other phenomena the planet had on its surface. With that knowledge, they hoped 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. The young Aquans planned to restore the shelters therein. (TAS: "The Ambergris Element")