Who is Matt Damon's Teenage 'Ab Double'?Academy Award winner Matt Damon is known to totally immerse himself in his roles, often undergoing brutal physical transformations in the process.

To play whistleblower Mark Whitacre in 2009's 'The Informant!,' Damon packed on a "horrifying" 30 pounds in no time.
"At my age, all I had to do to gain weight was eat the way I did when I was in college and the weight went on instantly," he told the NY Daily News.

Despite his portly presentation in the film, we're more used to seeing the father of three in fighting shape, so many audiences were shocked when "Mr. Damon's Abs Double" rolled across the credits at the end of 'True Grit.'

This guy was voted the Sexiest Man Alive by PEOPLE, and has spent a good part of his film career in a neon Speedo ('Talented Mr. Ripley') or kicking ass for justice in remarkably tight shirts in the 'Bourne' movies.

So who is this abdominally gifted actor and where was the Damon torso we've come to know and love?