On 3 August 2012, at the London Aquatics Centre, Katie Ledecky touched the wall first in the women's 800m freestyle. She was fifteen, the youngest member of the United States swim team, and swimming in her first Olympic final.
Seeded third, out front from the first lap
Ledecky had entered the final seeded only third, with an 8:23.84 from the heats. She ignored the ranking and pushed to the front of the pack straight away.
By the 200-metre mark she led the field and was swimming under world-record pace. She held that line until the final quarter, missing the record by less than a second on the last lap.
She finished in 8:14.63, a personal best that broke Janet Evans' 23-year-old U.S. record. Spain's Mireia Belmonte García took silver in 8:18.76, and Britain's defending champion Rebecca Adlington faded to bronze in 8:20.32.
A four-second win that ended a twelve-year American wait
The margin was nearly four seconds. It gave the United States its first Olympic 800m freestyle title since Brooke Bennett in 2000, and it came from a swimmer barely known outside the sport.
USA Swimming's national team director Frank Busch, watching the upset, told The Washington Post: "It's one of the biggest shocks I've ever seen in the Olympics. Stuff like this just doesn't happen."
The London race began a run of four consecutive Olympic golds in the event, through Rio, Tokyo and Paris. For Ledecky, the wall she touched at fifteen was the start of everything that followed.