When the porthole breaks, air will start to rush out into the sea, with some violence and a good deal of noise. The air pressure exceeds the water pressure at the porthole by an amount proportional to the height of the porthole above the moon pool water surface. This appears to be about 2.4 m (8 ft) and the overpressure will be 0.24 atmospheres (about 3.5 PSI). Even though Charlie has already closed the control room door, the air in the control room retains this overpressure until the porthole breaks. The air then rushes out into the sea and within a couple of seconds, its pressure reduces to match the water pressure at the porthole. The air will now escape more slowly in great big bubbles, and gulps of water will slosh in between bubbles, just as happens when you push an open empty bottle