This is a very interesting topic in mythbuster at discovery channel. This might be helpful while you encounter this situation and want to help yourself. I also saw this at Man vs Wild, another program in discovery channel, which told us to use hammer to break out the windows. And the mythbuster confirm this. lol....Here is the busted myth as follow: If a car falls into the water and becomes submerged, you cannot open the door until the interior is flooded. confirmed The pressure differential between outside and inside when the car is submerged is too great for a man to force the door, and the pressure must first be equalized, which means the interior must be flooded first. But it should be noted that Adam was forced to resort to emergency air in his first test. A second test later showed it to be possible to escape the car simply by opening the door, but only by remaining calm and not attempting to open the door until the interior is well and truly flooded, so as to conserve oxygen whi...