Unless you were hiding under a rock over the past six months or so, you know the Foo Fighters recorded their latest album, 'Sonic Highways,' in eight of America's biggest musical cities. The entire process was also documented for an HBO series of the same name. But that doesn't mean Dave Grohl is out of stories yet. He opened up about 'I Am a River' -- based on New York City -- in a video for 'The Tonight Show Starring Jimmy Fallon.'

In the web-exclusive 'How I Wrote That Song' (watch it above), Grohl gives a little background on the 'Sonic Highways' concept and explains how he would interview local music icons and begin incorporating phrases and references into the lyrics. "I would sort of cut and paste and fit them in," Grohl said. "So the next day, I would go sing the new lyric, and it was meant to represent that whole week that we'd been there by telling the stories of the people that I'd talk to."

He also spoke specifically about 'I Am a River,' the NYC-themed album closer. He said the lyric, "I found a secret behind a Soho door," is an homage to the Magic Shop recording studio and the river allusion comes from Minetta Creek which runs underneath Manhattan.

"This song is mostly about that, this river that runs underground through the city," Grohl said. "I thought it was a beautiful idea that there's something natural and prehistoric that runs underneath something as monolithic and futuristic as New York City. And maybe we're all connected by something like that."

The Foo Fighters will be taking 'Sonic Highways' on the road next summer in a massive tour that will also celebrate the band's 20th anniversary. Grohl also contributed to 12 tracks on the upcoming album by punk-metal supergroup Teenage Time Killers.

Check out the band's 'Tonight Show' performance of 'I Am a River' below.

Foo Fighters -- 'I Am a River' on 'The Tonight Show Starring Jimmy Fallon'

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