FOR A MOMENT there, the narrow balcony above the Ballet Vietnamese eatery verged on collapse. Twenty or so hyper partyers had sardined themselves onto the rickety perch, causing it to sag precariously over the heads of the herd—bodysurfing, hundreds deep—clogging Pike Street on a steamy Sunday evening last July. Neither they, the fans dangling over the edge of the rooftop above them, nor the yellow-shirted fanatic who shimmied up the utility pole, cared: The Dead Weather was performing not half a block away. On a stage butting against Broadway, amid spastic lights and a cranked sound system, raged the Kills’ Alison Mosshart, Jack White, and the rest of the supergroup. It was an epic rockout—and it went down in one of Seattle’s densest corridors.

To the uninitiated, this is the Capitol Hill Block Party. In June MTV dubbed it one of North America’s best under-the-radar summer blowouts. It’s our city’s most revelrous, claustrophobic, buzzy music festival and, thanks to the rowdy crowds, possibly its most polarizing.

This year the block party turns 15, and attendance is expected to approach 30,000 over three days, July 22 to 24, to see 70 acts on five stages.

The marquee people-packer has come a long way from the first Capitol Hill Block Party, a one-day DIY affair organized by Jen Gapay in 1997. Gapay, then 29, had moved from Montana where she’d worked in advertising for the alternative weekly Missoula Independent. Put off by the crowds and corporate vibe she thought had come to define Bumbershoot, she was hooked on the idea of partying in the open streets for free. She corralled five bands, a handful of DJs, and set up a small stage along 10th between Pike and Union. On the third Saturday of July, a couple hundred people partied to Flight to Mars and Mavis Piggot. When it was over, Gapay knew her scrappy fest had the potential to grow into something special.

For two years, Gapay slowly expanded the band roster and venue space. And then she got hired away to produce New York’s Siren Music Festival.

The Hill at the time was an alt hinterland dotted with a handful of bars, affordable apartments, and coffee shops, a far cry from the corridor now throbbing with clubs, restaurants, stores, condos, and businesses along Pike/Pine. The neighborhood became a natural landing pad for the eclectic crowd getting pushed out of Pioneer Square by the dot-com boom of the late ’90s. Marcus Charles, who owned Marcus’ Martini Heaven on Yesler Way, followed his customers over I-5 and opened Bad Juju Lounge in the 11th Ave space that now houses Purr.

In 2000, Charles manned the beer garden at the block party. That first year without its founder proved a colossal flop, so poorly organized that those involved pledged, “Not it!” for the following year. Charles, however, saw an opportunity.

He struck up a partnership with band manager Dave Meinert. Together they built on Gapay’s indie ideals but spun it as less a gathering and more an event celebrating the neighborhood. They enlisted sponsorship from The Stranger, charged eight bucks a ticket, packed a roster of 20-plus bands headlined by Mudhoney, and recruited pro-music politicians to speak. That year, Block Party drew raves from the nearly 3,000 in attendance, its biggest crowd yet. A report from the KEXP tent described how lucky crowds were to catch “one of the nation’s most talked-about live band[s],” Portland trio the Gossip. “I thought the guitar player was going to knock the entire tent down,” the author wrote, adding the performance “left everyone standing with their jaws hanging wide open.”

Such moments, promising the discovery of something new and notable, struck a chord with music-obsessed Seattle. More memorable moments followed—that time Sir Mix-a-Lot did “Baby Got Back” while blue- and purple-haired hipsters danced with him onstage, the reunion of hometown late-’90s garage punk band Murder City Devils in 2006, Sonic Youth’s rare festival appearance in 2009.

In 2010, attendance notched a new record: 30,000. For the first time the city’s special events office granted organizers a one-day extension on a trial basis, a request partly prompted by the Dead Weather (the headliners whose fans nearly brought down that balcony). The group approached bookers about playing Block Party, but could only swing a Sunday showing. And when a group of that caliber knocks, you open the door.

But unlike other fests the Weather might play—like, say, Chicago’s sprawling Lollapalooza, where it takes longer to trek from one stage to another than it takes a Seattle festgoer to hit several restaurants, a tattoo parlor, and a hot dog stand—Capitol Hill Block Party isn’t cordoned off in a field or a park. It’s in the heart of the new heart of the city. And shocker: Three days of congested revelry pisses some people off.

Though bars and restaurants within the party perimeters inevitably make bank, some retailers have reported substantial dips in sales, despite the guaranteed foot traffic. At community meetings last winter party organizers heard from those merchants as well as the property manager who’d been circulating antiparty petitions out of concern for her aging building. And the bookseller who spotted interlopers on his roof attempting to sneak into the festival. And the Ferrari dealer whose showroom was not only blocked by unwieldy lines but was also a pit stop for a drunken partyer engaging in a solitary sexual act. They all had one question: How will you make sure the block party doesn’t mess up my business?

The protestations threatened the future of CHBP. Rumors swirled that the city would nix a third day if not the entire festival. But in mid-April organizers got the go-ahead for another three-day fest, but only after agreeing to sell 500 fewer tickets a day and some extensive outreach: Block Party heads have worked with retailers to coordinate deals to draw in festgoers. Its website spotlights Hill businesses. And Block Party promised to contribute more dollars toward police presence and cleanup. Square Room, the art gallery and home store, which had reported thousands of dollars in lost revenue during past block parties, will serve as an artists’ lounge. Some proceeds will go to nonprofits like the Capitol Hill Housing Foundation. Barring a resurgence in the opposition, the block party will be a three-day shindig from here on out.

Still, one question remains: Will the Capitol Hill Block Party outgrow the neighborhood? Charles and Meinert both shrug. “Does the city view Block Party as something it wants? Does it view it as a pain in the ass?” Meinert asks, pointing out that the Hill in which they got their start no longer exists. “The challenge is that the neighborhood has evolved. Every neighborhood evolves. As it does, I think we need to be conscious of that evolution and how we fit into it,” says Meinert. “And the neighborhood needs to do the same thing.”