Answer:
During the time of the supercontinent Pangaea, there was a unique enormous sea, Panthalassa.
Panthalassa was an ocean that surrounded the Pangea continent during the Paleozoic and Mesozoic, being the only ocean on Earth at that time. At Panthalassa, two great sea currents circled the poles from the equator.
As Pangea disintegrated into parts that moved farther apart, the original Panthalassa remained only in its central part, the present Pacific. Other current oceans have emerged between the continents that have separated from Pangea.