It very definitelyh is NOT a theory at all. Just an imaginary legend.
A theory is the starting point for evaluation by the Scientific Method in which experiments are devised to provide evidence that it is false. When numerous experiments fail to prove it false, the validity of the theory becomes more evident, and additional and more powerful experiments may prove conclusively that the theory is true. This is how scientific knowledge is derived and why it is trusted.
The Hegelian dialectic may be applied as well: thesis, antithesis, synthesis. The initial idea is presented, equal and opposite ideas are offered in its place for a logical critique, and a final synthesis, or theory, arises after a considered investigation using the original thesis and the antithesis.
A myth is something that people have been led to believe, but often without a clear definition and without any plausible explanation except beliefs. Most myths are false by their very nature.
However, many of what were once considered myths of the ancient world have been or are being located. For instance the fabled Hanging Gardens of Babylon are believed to have been located, not in Babylon, but north in Nineveh by Dr. Stephanie Dalley of the Oriental Institute at England’s Oxford University. Other one time myths, like the great lighthouse on the Island at Pharos, evidence of which are slowly being assembled. At the bottom of the light was a platform filled with statues that are still being discovered by present-day divers off the coast of Alexandria.
Yesterday's myths, in some cases, are becoming today's reality.