Katie Ledecky Breaks 400m Freestyle World Record

Katie Ledecky reacts after her heat win in the Women's 400 Meter Freestyle Prelims during the 2014 Phillips 66 National Championships at the Woollett Aquatic Center on August 9 in Irvine, Calif. ( Harry How / Getty Images

Ledecky, 17, clocked 3:58.86 to beat Italian Federica Pellegrini‘s mark of 3:59.15 set at the 2009 World Championships.

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