NEW YORK (AP) — If you listen closely to Tom Morello's new album, you'll hear a 24-year-old guitar riff. He's been patiently waiting since the mid-1990s to finally unleash it.

The song "Vigilante Nocturno" contains a riff he wrote during recording sessions for Rage Against the Machine's "Evil Empire" but never found its way into a song. So it went into Morello's stockpile.

"It didn't find footing then, but I always kept that one in my back pocket. Like, 'That is a badass riff and one day it's going to tear people apart,'" he said.

That day is now with the release of "The Atlas Underground," which finds Morello teaming up with an eclectic collection of artists he hopes will "challenge the conventions of rock 'n' roll and electronic music and hip hop and punk."

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Morello, who is listed among the 100 greatest guitarists by Rolling Stone magazine, has collaborated with folkie Marcus Mumford, alt-rock darlings Portugal. The Man, guitar god Gary Clark Jr., the Wu-Tang Clan's RZA and GZA, rapper Big Boi and the hypnotic DJ Bassnectar, among others.

"The album features artists of diverse genres, ethnicities, ages and genders and that, in itself, is a statement of these divisive times," said Morello. "The idea was to forge a sonic conspiracy" and "make a new genre of rock 'n' roll."

Morello, who is a fan of Mumford & Sons and has performed with the band several times, asked lead singer Mumford for help on the song "Find Another Way." Both are rock dads who carved out time to collaborate via Skype from different time zones.

"So he would put his kid to bed in England and I would drop my kids off at school in Los Angeles and then return home to FaceTime with our acoustic guitars out and write that song," he explained.

As for Clark, he stopped by for a "three-hour blues-rock explosion jam" that was honed down to a "concise freight-train-disco demolition-metal thing" that would fit "very comfortably both in a mosh pit and the dance floor." It's called "Where It's at Ain't What It Is."

Morello, 54, is known for his shredding chops — he was in Rage Against the Machine, Audioslave and Prophets of Rage — but he stretched sonically, too. Even into electronic dance music.

"I was never a fan of EDM music, or at least my understanding of it until I was introduced to Knife Party and I felt all of the aggression and the tension and release of the greatest rock 'n' roll music in their catalog," he said.

Morello also highlights up-and-coming artists, like exciting alt-rocker K. Flay, born Kristine Flaherty. He was driving his kids to school when he heard her soaring, Grammy-nominated song "Blood in the Cut" on the radio.

"I pulled the car over. I got on my phone and I texted my manager: 'Find me K. Flay. I don't know if it's a man, a woman, a band, an orchestra — whatever K. Flay is I need to be in business with it,'" Morello said. "And it turns out that she's fantastic."

For her part, Flaherty recalls being shocked when she got an email from Morello proposing they work together. She and her guitarist were on the road and they'd get pumped each night watching Rage Against the Machine performance footage videos on YouTube.

"It's kind of the ultimate honor for me, not only because Tom is a legend and a pioneer and every other noun you could use in that world, but he's also just a good person and a creative person and someone who likes making things for the sake of making them," she said. "Those are the people that I want to be surrounded by."

It wouldn't be a Morello record without the lyrics grappling with social issues and "The Atlas Underground," out Friday, has songs that deal with police violence, structural poverty and racism. He says his political activism was born from being the only African-American in his Chicago suburb.

"There was a noose in my family's garage when I was 13 years old and in the town we lived in there was a burned cross and stuff," he said. His mom taught at a public high school and sometimes the things written on her chalkboard "would it be straight out of 'Mississippi Burning.'"

He credits his mom — now 95 — with introducing him to Malcolm X and instilling in him the need to stand up against injustice everywhere. He was late to learn the guitar — 17, so he practiced eight hours a day —and then went to Harvard University to study political science. "With those two fists, I was going to march into the world to do as much damage as I could."

He's regularly highlighted advocacy groups along the way that have helped the homeless or prisoners. This time Morello is partnering with Color of Change to raise awareness about police brutality and race relations in America.

"If I have one message for the people in my audience is that, 'You are agents of history. When the world has been changed in the past, it was changed by people with no more courage, power or smarts than you have,'" he said. "It's a matter of standing up in your time for justice and for what you believe is right."

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Mark Kennedy is at http://twitter.com/KennedyTwits