Heavens
When I first started reading this verse, I thought that the "expanse" or "firmament" was going to be land. But it actually ends up being the heavens. Some others have speculated as I have that the atmosphere of the earth was very different before the flood. The "canopy" some call it. It is probable that the canopy is what allowed the garden of eden to exist, what allowed men to live for so much longer, and what ultimately fell to earth during the great flood of Noah's time.
