American 15-year-old Katie Ledecky stunned Rebecca Adlington to take the women's 800m freestyle Olympic title, with the British favourite third.
Ledecky took off at a stunning pace and maintained her lead, leaving defending champion Adlington and Danish rival Lotte Friis floundering.
Ledecky finished in eight minutes 14.63 seconds, four seconds clear of second-placed Spaniard Mireia Belmonte Garcia.
"I think we all feel a little deflated after watching that, we wanted to see a gold medal from Rebecca Adlington, especially in this pool.
"Katie Ledecky was the unpredictable factor and I think that speed up front could have shocked Rebecca."
Adlington finished in 8:20.32 for her second bronze medal of London 2012.
"I am proud to get a bronze, there is nothing to be embarrassed about," Adlington, the 2011 world champion, told BBC Sport.
"I hate it when people say it is losing because you have not done my sport. Swimming is one of the hardest events to get a medal at.
"It's not like other sports. Hopefully the public will be proud of me getting that bronze."
There was nothing the Briton or Friis, the fastest qualifiers and pre-race leading candidates, could do to arrest Ledecky's phenomenal pace. Friis crept home in fifth.
'I am proud to get a bronze, there is nothing to be embarrassed about," says Adlington
Adlington swam to 400m bronze earlier in the week in a time of 4:03.01. Here, Ledecky ripped through 400m in 4:04.34 - just a second slower - en route to gold over twice the distance.
By that point, Adlington was only a second back, but this was a far faster start than she might have expected. She faded as Ledecky, eight years her junior, pressed home her advantage.
Friis and Adlington are used to racing each other and know each other's strategies. Ledecky, the unknown quantity, met the pair for the first time in a major final and turned the element of surprise into Olympic gold.
Ledecky is the youngest Olympian on the 529-strong American team and the second-youngest Olympic swimming medallist in US history.
Adlington sliced almost a second and a half off her qualifying time as she tried to deliver gold for Britain, but Ledecky found a staggering nine-second drop from her own heat.