Myth or reality? Other than the fact that such a geographical region exists, it's a myth. Proper investigations into the so-called �mystery' invariably show that perfectly reasonable explanations cover the phenomena.
For example, one such investigation into the disappearance of a whole flight of aeroplanes which went missing was carried out by a friend of mine who works for the Air Accident Investigation Board. His evidence was shown in a TV documentary in 2004. He proved conclusively that no supernatural explanation was needed, as the error was on the part of the leader of the flight. He imagined his compass was malfunctioning - as was made clear in conversations with the control tower - and then overflew their first turning-point in a triangular training exercise. Thereafter, he mistook one island for another, because of their similar shapes, which further compounded their mistakes. Eventually, they simply flew out over the Atlantic until they ran out of fuel and crashed.
Seabed gases - sometimes blamed for missing ships - are extremely unlikely to affect aircraft.