American teenager Katie Ledecky smashed the world record in the women's 1,500 metres freestyle final by more than six seconds during a glittering session for the United States at the world championships.
The 16-year-old from Bethesda, who also won the 400 freestyle gold medal last Sunday, powered away from defending champion Lotte Friis of Denmark in the final stages to break the previous best of 15:42.54 set by compatriot Kate Ziegler in 2007.
It was the second world record of the championships in the specially constructed pool at the hilltop Palau Sant Jordi after Ruta Meilutyte's mark in qualifying for the 100 metres breaststroke on Monday.
Lithuanian Meilutyte, born two days after Ledecky in March 1997, went on to claim a first world championship gold for the tiny Baltic nation when she triumphed in Tuesday's final.
She was unable to better Monday's effort, but still produced the second-fastest time yet of 1:04.42, just seven hundredths slower than her record 1:04.35.
Ledecky's teammate Missy Franklin, who won four gold medals and one bronze in last year's London Olympics, secured her second gold of the week with victory in the 100 metres breaststroke and Matt Grevers claimed the 100 metres backstroke title.
Ryan Lochte, the second-most decorated men's world championship medallist after compatriot Michael Phelps, provided a rare moment of disappointment for the US team when he could manage only fourth in the 200 metres freestyle final.
Frenchman Yannick Agnel followed up on his triumph at last year's London Olympics to take gold and Conor Dwyer salvaged some pride for the US with silver.
"We've had an absolutely incredible evening," an excited Franklin, who is swimming eight events in Barcelona, said.
Ledecky sat on the shoulder of Friis for much of their two-horse race, but the Dane was left trailing by a powerful late charge from the Olympic 800 metres champion and finished with silver in 15:38.88, also inside Ziegler's mark.
Lauren Boyle of New Zealand was third in 15:44.71 ahead of Spain's Mireia Belmonte Garcia in 15:58.83.
"It was really tough, my hardest race ever," Ledecky said. "I knew we were going pretty fast and I figured that whoever came out on top was probably going to get the world record.
"I had to be careful not to push it too early or too late and just touch the wall first. Around the last 200 I knew I could take off."
Succeeding Ziegler as the fastest woman over 1,500 metres was extra special, Ledecky said, as the 2011 world championship silver medallist behind Friis hails from the same part of the US. "It means the world to me," said a still-dripping Ledecky, goggle marks visible around her eyes.
"Kate Ziegler is from my area and I have looked up to her my whole life. I am really honoured to break that world record and to keep it in the Potomac valley. I'm still 16, so I hope I have plenty years left of swimming and I guess we'll see where my limit is."
Franklin, 18, had a gruelling day as she had to qualify for last night's 200 metres freestyle final around an hour after claiming backstroke gold.
It was her fifth world championship title, already putting her fourth among her compatriots.
"It was tough but it was awesome," she said. "It was nice to have the final before the semi-final - it was the other way around in London," she added.
Agnel added a world 200 metres freestyle title to his Olympic gold when he outgunned Dwyer, with Russia's Danila Izotov taking bronze ahead of Lochte.
"This medal means a lot," said Agnel, who has recently started training with Phelps' former coach Bob Bowman. "I just wanted to enjoy myself, I was relaxed. Even if a world title is less important than an Olympic title this is hugely significant."