No one is sure who this person really was, although he certainly did exist. Over the course of centuries, over 4000 texts have been written on the subject. Each author has had his own unique opinion, and to date there are roughly 60 people who have been suspected of being the Man in the Iron Mask.
I visited the island of St. Marguerite in France this summer, where there is a prison in which this prisoner lived for 11 years before being transfered to variuous other prisons. I visited his prison cell, and can safely say that I can not think of a much bleaker existence than the one which he lived. He lived in a small, rock chamberthat contained a single window. This window contained three sets of iron grates, the guide explained, not only so that the prisoner could not escape, but also so that he could never throw a message out the window, or in any way communicate with the outside world. His identity was such a closely-guarded secret that after he died in the Bastille, his cell in St. Marguerite was cleaned thoroughly, and every piece of furntiute in the room was burned, so that if, for example, he had ever carved a message into his bed, no one would find it.
Also, a point of clarification should be made that his mask was not really made out of iron, but rather of leather, and furthermore, he only wore this mask when he was being transferred from one prison to another.
There is a local legend near the island of St. Marguerite that the prisoner managed to escape and lived in the nearby city of Antibes for several days before he was captured. During this time, the legend says, he lived with a young women. The king, fearing that this woman knew the identity of the prisoner, had her locked away in a prison in Italy. When she gave birth to a bastard child in prison nine months after the prisoner's escape, she had to fill out a birth certificate. Since she could not write the father's name, she said that he was of noble birth (in French "de bonne partie"). Translated into Italian by the guards of the prison, this read as "Bonaparte." According to the local legend, this masked prisoner was the father of Napolean Bonaparte.