Kacey Deamer for Live Science

A 350-year-old French mystery has been unmasked: In his new book, Paul Sonnino, a professor of history at the University of California, Santa Barbara, claims he has uncovered the real identity of the mysterious Man in the Iron Mask.

The Man in the Iron Mask was a prisoner arrested in 1669 and held in the Bastille and other French jails for more than three decades, until his death in 1703. His identity has been an enduring mystery because, throughout his imprisonment, the man's face was hidden by a mask, according to Sonnino. The story was even popularized in the 1998 film "The Man in the Iron Mask," starring Leonardo DiCaprio.

It's a mystery that evaded even famed philosopher Voltaire and writer Alexandre Dumas. Historians have discounted the theory popularized by Voltaire and Dumas that the masked man was the twin brother of Louis XIV, according to Sonnino. [Bones with Names: Long-Dead Bodies Archaeologists Have Identified]