Paul McCartney and surviving Nirvana musicians Dave Grohl, Krist Novoselic and Pat Smear made history last year for the '12-12-12' Hurricane Sandy benefit concert. More than seven months later, the four musicians united again during Paul McCartney's July 19 concert in Seattle.

Shortly after McCartney, Grohl, Novoselic and Smear created the song 'Cut Me Some Slack' during an impromptu jam session, the quartet performed it at the '12-12-12' concert and on an episode of 'Saturday Night Live.' Rumors of a Nirvana reunion with Sir Paul at the helm made the rounds, but the idea was quickly swatted down by the quartet. 'Cut Me Some Slack' was released on Dave Grohl's 'Sound City' documentary soundtrack and that was the last we had heard from 'Sirvana,' until July 19, 2013.

"We hatched that plan a couple of weeks ago," McCartney tells Rolling Stone about the latest concert surprise. "Dave was going to come up to Seattle with his family, and I said to him, 'Do you think there's any chance the guys would fancy [a reunion]?' He said, 'Well, let me ask.' And they fancied it. So we thought, 'Let's go and flip 'em out.'" The group took the stage during McCartney's first encore to perform 'Cut Me Some Slack' along with the Beatles classics 'Day Tripper' and 'Get Back.' They returned during the second encore, jamming on 'Long Tall Sally,' 'Helter Skelter' and 'The End.'

"When you're playing with guys like that, you realize that they've been to the heights of fame, so they're used to it – they're not gonna freak," says McCartney. "And they're hell of musicians, so we learned everything really quickly. That's what's great about playing with guys like that … "They're powerful, you know. It's a great power to have them on stage with you. I mean, my band's great – but when you augment it with Nirvana, that's greater."

Check out these fan-filmed videos of Sirvana's Seattle reunion below!

'Cut Me Some Slack'

'Helter Skelter'