Ledecky kicked off the first wall with a lead of nearly a body length and steadily pulled away from the overmatched field -- as well as the world-record line superimposed on the video screen as her powerful arms churned through the water.
When Ledecky saw the time -- 3 minutes, 56.46 seconds -- she let out an uncharacteristic scream and pumped her right fist. The 19-year-old crushed the mark of 3:58.37 that she set nearly two years ago on the Gold Coast of Australia, and had been chasing ever since.
Jazz Carlin of Britain took the silver and Leah Smith of the United States grabbed the bronze.