British swimmer Rebecca Adlington had to settle for a second bronze as 15-year-old American Katie Ledecky produced the second fastest 800 metres freestyle in history to take gold in the event.
Ledecky led from start to finish as the expected duel between defending champion Adlington and Denmark's Lotte Friis failed to materialise.
Instead, Ledecky went out fast and moved further ahead from the halfway stage, under world record pace all the way until the final metres to touch in 8 minutes and 14.63 seconds.
Only Adlington has ever having gone faster when she set a world record when taking the title in Beijing four years ago.
With 200 metres to go, Spain's Mireia Belmonte Garcia overhauled Adlington and finished second in 8:18.76. Following her 400m bronze, Adlington was beaten into third again and touched in 8:20.32.
"It was such a difficult race, she swam so fast. She (Ledecky) swam absolutely incredible," Adlington said after the race.
"I think she's only really young as well - 15 - it's amazing. The 800 was my event. I think the pressure and everything, the expectation, everything going into this meet has been a little bit of a battle but I'm so pleased with that.
"I would have liked the time to have been a bit quicker, I'm not going to lie. I've done that time all year and I don't know what happened. Everything just kind of caught up with me.
"I gave it my absolute all and I'm sorry that I didn't get the gold for everyone that was expecting me to.
"But I am so proud and pleased to get a bronze medal - it's nothing ever to be embarrassed about. I hate it when people say that silver or bronze is losing because you have not done my sport.
"Swimming is one of the hardest sports to medal at. We're not like other sports. It is so, so difficult and I hope the public realise this this week and hopefully will be proud of me for getting that bronze."
Earlier in the evening Britain's Lizzie Simmonds was an agonising fourth as 17-year-old Missy Franklin won the 200m backstroke in a new world record.
Simmonds, who was sixth in the event in Beijing and has appeared in two world finals, was swimming from lane one after only qualifying in seventh.
The Loughborough ITC swimmer went out fast and was second at halfway, although she slipped back into third with one length to swim.
And she was overtaken by American Elizabeth Beisel in the final 25m to be edged out of the medals in 2 minutes 07.26 seconds.
Franklin lowered Kirsty Coventry's world record to set a new mark of 2:04.06 and claim her third gold medal of the London Games.