 PLOT: SPOILER: As Cutter is explaining the props in Angiers show to  the judge, he demonstrates the sliding panel that allows the  occupant of the water tank to reach the lock, and points out that  the trick lock had been replaced with a real one. But this sliding  panel means that the top six inches of the tank would not be water  tight, and there would be large pocket of air to breath, not to  mention the opening created by the secret panel itself.