A study done in 1998 suggests that oceans may have appeared in the Hadean eon , about 200 million years after the earth had formed . According to the study , it was very hot then ( about 100 Â C at the start , dropping to 70 Â C during the Hadean ) . Morse and MacKenzie have suggested that oceans may have appeared first in the Hadean eon , as soon as two hundred million years ( 200 Ma ) after the Earth was formed , in a hot 100 Â reducing environment , and that the pH of about 5.8 rose rapidly towards neutral .