Could be that we're surrounded by water:
Genesis
1:6 And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.
1:7 And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament: and it was so.
1:8 And God called the firmament Heaven. And the evening and the morning were the second day.