Phelps wins 18th gold medal in final race of his career
London Olympics Swimming Men
United States' men's 4 X 100-meter medley relay team from left, Matthew Grevers, Michael Phelps and Brendan Hansen shake hands with teammate Nathan Adrian after winning the gold medal at the Aquatics Centre in the Olympic Park during the 2012 Summer Olympics in London, Saturday, Aug. 4, 2012. (AP Photo/Michael Sohn)