Air is the absence of a block. It was created in the game very early. There are no graphics at all on air and it can only be destroyed or created by placing or destroying blocks. Air is the only space the player and other Mobs are allowed to move, in Alpha mode entering a block will suffocate whatever is inside. Air will restore all bubbles on the Air meter and make it dissapear.
In current Creative mode and Survival mode, water is infinite and will fill any air that is horizontal to it or below it. In Indev, if a pool of water comes in contact with air next to it, there is a 2/3 chance the water will turn to another air block and a 1/3 chance the air will turn into water.
Non-solid blocks such as Mushrooms, Redstone wire and Flowers allow the player to walk through them and can breathe in them as if they were air.
Due to air's ability to be located on the map anywhere, it must be treated as a block by map editing software.