Teenager Ruta Meilutyte won Lithuania's first ever Olympic gold in the pool by beating American Rebecca Soni in the women's 100m breaststroke.
The 15-year-old, who goes to the same Plymouth school as Tom Daley, led from the start and beat the fast-finishing Soni by 0.08 seconds.
Japan's Satomi Suzuki earned bronze, while Australia's defending champion Leisel Jones finished fifth.
"I can't believe it," said a stunned Meilutyte. "It's too much for me."
Having set a new European record in the heats - a mark that beat her personal best by two seconds - Meilutyte swam even faster in the semi-finals on Sunday to qualify fastest for the final ahead of world champion and Beijing silver medallist Soni.
In the final, following a fault at the start, where the bleep went before the swimmers were told to take their marks, Meilutyte astonished both herself and the crowd to hold off her challengers and win in 1:05.47.
The gold was Lithuania's first medal of the London Olympics.
Meilutyte, who moved to Britain three years ago with her father, is trained by English coach Jon Rudd.
He said that a fast swim had been "brewing for a while".
"We didn't realistically know what she would do," said Rudd, who revealed that Meilutyte was a freestyler when she arrived in the United Kingdom.
"Her breaststroke was pretty tidy and we tidied it up even more.
"A big swim around the 1:05-1:06 mark was brewing for a while, it was just about putting the elements together on the day.
"She is a talented and vigilant worker. When you've got talent and work ethic you've got a great kid."
Meilutyte could add to her medal tally as she is competing in the women's 50m and 100m freestyle.
Plymouth College assistant headteacher Sarah Dunn said of her student's triumph: "We are almost in tears, it was such an achievement.
"We knew she had great potential when she arrived here three years ago. Not only is she a fantastic athlete, but she is also a great scholar.
"She has always demonstrated superb talent and the hunger and desire to succeed. We had every confidence in her.
"To achieve what she has at such a young age is incredible. She is a real credit and we are so proud of her."