Which is called mega ocean?

The mega-ocean was called PANTHALASSA, meaning all water. According to Wegener, all the continents formed a single continental mass and mega ocean surrounded the same. The supercontinent was named PANGAEA, which meant all earth. He argued that, around 200 million years ago, the supercontinent, Pangaea, began to split. Pangaea first broke into two large continental masses as Laurasia and Gondwanaland forming the northern and southern components respectively. Subsequently, Laurasia and Gondwanaland continued to break into various smaller continents that exist today. 

Further Reading:

• NCERT Notes: Continental Drift [Geography Notes For UPSC]
• The Origin And The Evolution Of The Earth [Geography Notes For UPSC]