Ireland's Kevin O'Brien hit the fastest hundred in World Cup history to sentence shell-shocked England to an astounding three-wicket defeat in Bangalore.
O'Brien (113) came to the crease when damage limitation seemed the best his team could hope for, after they lost three wickets for eight runs to falter to 111 for five in apparently vain pursuit of 327 for eight.
But he responded with an innings of immense power, smashing six sixes and 13 fours in a 50-ball hundred and dominating a stand of 162 in 17 overs with Alex Cusack (47).
No one in the England attack was safe, with Graeme Swann (three for 47) the lucky one that he managed to complete his full 10-over allocation before O'Brien began his assault in earnest.
England did not help themselves, dropping five catches in all, including O'Brien and Cusack once each.
In the end even O'Brien's run-out, trying to scamper a second run at the start of the 49th over, could not stop Ireland getting home with five balls to spare.
After O'Brien had very nearly finished England off, John Mooney clipped the winning four to midwicket in company with Trent Johnston.
Jonathan Trott (92) and Ian Bell (81) had been England's main batting contributors, the former also bagging a place in cricket history as the joint-quickest batsman to 1,000 one-day international runs.
But he was relegated from headline to footnote by O'Brien's unexpected heroics.
Ireland's prospects of pulling off a tournament record chase under lights appeared decidedly slim, and all the more so after they lost captain Will Porterfield to the first ball of their reply under lights.
They needed a good start yet got the polar opposite when Porterfield inside-edged an attempted cover-drive at a wide ball from James Anderson back on to his stumps.
Paul Stirling and Ed Joyce gave England, and Stuart Broad in particular, something to think about in a second-wicket stand of 62.
But Stirling got underneath a pull at Broad's replacement Tim Bresnan and was caught at deep square-leg in the 10th over.
Joyce then mis-pulled successive Bresnan deliveries and could easily have gone to either, on 21.
But first of all, Anderson's lunge in from midwicket was not quick enough - and then wicketkeeper Matt Prior spilled a high, swirling chance.
New batsman Niall O'Brien was dropped on five at long-off, where Anderson parried a big hit at Swann for six.
The consolation for England seemed to be that a repeat of the fielding mishaps they endured in their first match against Holland would surely not matter too much tonight.
So it still seemed when Niall O'Brien missed a slog-sweep at Swann, who had a second wicket when Joyce went up the pitch and was easily stumped after failing to cover the spin.
Swann was in business again when Gary Wilson was trapped lbw sweeping. But that merely brought O'Brien and Cusack together for what turned out to be the match-turning partnership.
Many huge blows had already been inflicted by the time Andrew Strauss dropped a tough skier, running back behind the bowler from extra-cover after O'Brien brutalised a ball from Paul Collingwood skywards on 91.
Cusack then escaped a tough caught-and-bowled chance to Mike Yardy on 32.
England finally got the breakthrough when Cusack failed to regain his ground after a mix-up over a single. But O'Brien knew he had done all the hard work, and was in no mood to give England a route back into a match they had once appeared to be coasting.
Trott had earlier passed his 1,000 ODI runs in only his 21st innings, apparently serving his team well with an exact run-a-ball tempo. He shared a third-wicket stand of 167 with Bell, after Kevin Pietersen (59) and Strauss had predictably got England off to a blistering start.
Strauss, who won the toss, and Pietersen immediately set about the opposition's new-ball pair Boyd Rankin and Johnston. The in-form captain was first to go, after a stand of 91 in less than 14 overs, when he tried to paddle-pull slow left-armer George Dockrell off his stumps and was bowled round his legs.
It was an odd shot, and Pietersen also went tamely - soon after reaching his first half-century as an ODI opener, from only 41 balls, when he attempted to reverse-sweep off-spinner Stirling but somehow presented a simple catch to wicketkeeper Niall O'Brien.
Trott and Bell therefore had to start again, yet lost little noticeable momentum doing so.
Neither gave a chance, Trott profiting again from his risk-free accumulation to help keep England above six-an-over almost throughout.
He survived Bell, who chipped a full toss low to a tumbling Stirling at straight midwicket off Mooney.
There were still 55 runs in powerplay. But after Trott also went short of his hundred, bowled aiming a big hit at Mooney (four for 63), England faltered and could manage only an under-par 70 in the last 10 overs.
Former Ireland captain Johnston kept tight lines to bowl Prior and Yardy and become the first bowler to take 50 ODI wickets for his country.
England had ended their innings with a whimper, but it seemed highly unlikely to cost them - until O'Brien's muscular and mesmeric intervention.
The Fine Gael leader Enda Kenny paid tribute to the Irish team. He said: 'Ireland’s cricketers exemplified all the best qualities of our nation, playing with courage and pride and showing that seemingly insurmountable odds can be overcome.
The victory marks one of the finest days in Irish sport and my warmest congratulations go out to each and every member of the squad after this historic win.
'Their supreme effort will lift the spirits of every single Irish person, no matter where they are in the world. Ireland’s performance is truly inspiring demonstrating that, with self-belief, the apparently impossible can be made possible and real change can occur.
'All members of the Irish team played an integral role in this historic triumph and I wish the team every success in the remainder of the tournament.
'I look forward to greeting the players upon their return home.'