Answer: Water vapor
The condensation of water vapor formed the early oceans. It is an important part of the water cycle.
The oceans on earth formed around billions of years ago. The water was in the form of water vapor until the temperature of the early earth dropped below 212° Fahrenheit. The water present along with the other reducing gases like methane, hydrogen, carbon dioxide on early earth atmosphere condensed in the form of rain. As, the water drained into the great depressions of the earth's surface, the primitive oceans were formed. The formation of ocean contributed towards the water cycle.