What is pan thalassa?

Panthalassa (from Greek πᾶν (pan) "all"; and θάλασσα (thalassa) "sea") was the vast ocean that surrounded the supercontinent of Pangaea. It was the only global ocean during the late Paleozoic and early Mesozoic eras.

Panthalassa began to shrink about 200 million years ago as Pangaea began to break apart. As Pangaea continued to break up, Panthalassa was divided into smaller oceans: the Pacific, Atlantic, and Indian Oceans.

The geologic history of Panthalassa involved the subduction of oceanic plates, continental collisions and the formation, subduction and separation of numerous microcontinents. As one ocean basin closed due to subduction of the oceanic crust, another ocean opened up somewhere else. Panthalassa completely closed, and the modern oceanic basins came into existence only about 150 million years ago.