Air does not generate in caves
I discovered this bug when trying to fill in my caves with stone with world edit
Trying to //replace air stone was not working. Experimenting, I discovered that:
Placing and breaking a block allows that block of air to be replaced
Marking position 1 outside a cave and marking position 2 inside one and trying to replace air with a block will show you the boundary of the cave.
I figured this was a minecraft problem because I don't see any way world edit could distinguish between cave air blocks and above ground air blocks.

Edit: Apparently "cave air" exists, and that was probably the cause of my problem. I don't know how to close bug reports, so a mod is gonna have to do that for me.