http://bible.cc/genesis/1-7.htm. The firmament has a few definitions, including the heavens, the air in which birds fly, and is also known as the expanse.
Ping
As water enters the mantle(and it does through subduction zones) it becomes supercritical, meaning in a geseous/liquid state. It would penetrate back up through the mantle rock realizing its liquid state through underwater volcanism and other means. Since the core of the earth is iron and a supercritical liquid is lighter, it will always naturally reenter the most stable state.
Now there’s some science for you.