During Creation, the division of the waters by the firmament

And God said, Let there be a firmament [expanse] in the midst of the waters, and let it divide the waters from the waters.
[Related link:  http://www.washingtonpost.com/national/health-science/clue-to-how-our-oceans-were-filled/2011/10/20/gIQAmBRs0L_story.html]