Pundits are asking how, for 50 metres, China's 16-year-old new women's 400m individual medley champion swam faster than her men's counterpart.
Ye Shiwen, who wasn't among the pre-race favourites, announced her arrival on swimming's greatest stage with a world record in the gruelling 400 individual medley.
Her time of 4 minutes, 28.3 seconds beat the world record by more than a second and her previous best by 5 seconds.
Her final length of 28.93 seconds topped Ryan Lochte, 27, of USA, who swam the last length in 29.1 seconds in his gold medal winning race in the men's 400 individual medley. His overall time was 4min, 5.18 seconds.
The reaction to the swim in the BBC commentary box caused a controversy on Twitter, the Daily Mail reported.
BBC presenter Clare Balding asked former British Olympian Mark Foster, in the studio as a pundit: ''How many questions will there be, Mark, about somebody who can suddenly swim so much faster than she has ever swum before?''
Chinese swimming has previously been tainted by drug scandals – another 16-year-old world champion tested positive for doping last month – but Foster sought to play down any suggestion of cheating.
He said: ''It was a five-second best time and it was the way she did it as well. Bearing in mind she is 16 years of age, and when you are young you do some big best times… it can be done.''
Balding's question provoked a storm among BBC viewers on Twitter, with many praising her for daring to even hint at the possibility of cheating, but many criticising her for tainting the Chinese swimmer's achievement and some even calling for her sacking.
Ye herself said: "I'm still young. After training I'm sure I'll be even better."
"This is a big step for Chinese swimming," Ye said.
Local hope Hannah Miley of Britain settled for fifth and was marvelling at her Chinese rival's speed.
"That was very, very fast, and it'll be intriguing to look at the analysis and stuff because there are a lot of things that they've obviously taken from us and learned from," Miley said. "And, for us, it's an opportunity to learn from them."
Lochte said Shiwen's swim was a talking point, Yahoo Sports reported.
"We were all talking about that at dinner last night," Lochte said. "It was pretty impressive. And it was a female. She's fast. If she was there with me, I don't know, she might have beat me."
US men's swimming coach Gregg Troy said: "Heck of a swim. You notice stuff like that... You guys can do the research. I think that's probably the fastest women's split ever."