London 2012 - United States swimming gold medallists (L-R) Michael Phelps, Nathan Adrian, Matthew Grevers and Brendan Hansen send a message of thanks after the medal ceremony for the Men's 4x100m Medley Relay Final at the Aquatics Centre on August 4, 2012. The race marked the end of Phelps' Olympic career, during which time he won gold an astonishing 18 times.