During the last 15 years, the Ultra Music Festival has grown into what may stack up as the country's biggest dance music event: six days, with attendance topping 300,000 and $79 million flowing into the Miami economy.

In 1999, dance music in the United States was in an odd place.

On one hand, it was underground: U.K. rave had hit America, and illegal parties in warehouses, deserts and fields swept from coast to coast. On the other hand, dance-or "electronica," as it was then known-was having a mainstream moment, with Fatboy Slim picking up three MTV Video Music Awards behind "Praise You."

This was the lopsided but promising environment that welcomed the country's first legal, day-long, fully produced dance music event: the Ultra Music Festival.

"We wanted to do something unique and original, and none of us had ever heard of an electronic music festival on the beach with real production and real artists," says Ultra executive producer/CEO Russell Faisbach, then a local Miami promoter of warehouse parties that capped out at 2,000-4,000 attendees. That first event drew 10,000. It also exacted a toll: "It took everything I had, not just financially, but emotionally, physically, mentally," Faisbach says.

Fast-forward 15 years: Some of the same artists are still around (Fatboy is playing this year), there's still an underground, and there's a mainstream. But the electronic dance music explosion teased during the late '90s actually happened a decade later, not as electronica, but as EDM. Powered by a vibrant youth culture, EDM is a different, more powerful animal, touching rock, hip-hop and pop. It's the fuel of the post-genre era, and despite lean years, adversity and a host of competitors, Ultra is still its festival. The thriving event extended to two weekends, March 15-17 and 22-24, this year.

"It's different from other music festivals in that it's an entire immersion in the EDM scene, in every single thing," says Heineken brand director of sponsorships and activations Pattie Falch, who attended her first Ultra four years ago. (Heineken has been a sponsor for six.) "Having never been to something like that, it was an experience-from the music, to what people wear, to the entire environment." Heineken leads the pack of sponsors again this year (which includes Red Bull, SiriusXM, Don Julio tequila and Ciroc vodka), and branding experts estimate the Heineken deal at more than $1 million.

With the added weekend, Ultra could unseat its closest competitor, Electric Daisy Carnival, from the top EDM festival spot for the first time, and Florida-based consulting firm Washington Economics Group estimates it pumps $79 million per year into the Miami-Dade economy. EDC, a three-day Las Vegas event that drew 320,000 attendees last year, is primarily defined by its massive, bedazzled stages and trippy accouterments like amusement park rides and fire sculptures. It's also known for being on the market: Rumors have put it in the hands of Robert F.X. Sillerman's SFX Entertainment, Live Nation and AEG during the past year.

Ultra is more music-focused, making room for niches like deep house, minimal techno and trap. It added a Live stage for the first time in 2004, with acts like LCD Soundsystem and the Rapture-a risk that didn't immediately pay off. "I remember looking at that stage and thinking about all the money we invested in it, and there were four people standing in front of it," Faisbach says. But Live stage acts like M83 drew huge crowds last year, and the 2013 lineup includes Azealia Banks and the Weeknd.

And Ultra remains pointedly indie: It merged last year with Ultra Music--Patrick Moxey's label/full-service EDM company (the separate companies had spent some legal energy fighting over the name in years past)--as a means of expanding its capabilities and footprint, rather than take the acquisition bait. That hasn't necessarily dissuaded the suitors. "Certainly I have huge respect for Ultra and the people who are involved with it, so who knows?" Sillerman told Billboard in January, when SFX purchased two Miami-based nightlife promoters. "Maybe someday we'll be partners."

But with six additional festivals under the Ultra Worldwide banner (Argentina, Brazil, Chile, Croatia, Ibiza and Korea), a deal with YouTube to stream the flagship Miami event live and documentary film "Can U Feel It: The UMF Experience" soon available on iTunes, Ultra says it's doing fine on its own. "Sometimes throwing money at something doesn't make it any better," says Adam Russakoff, Ultra's executive producer, talent booker and director of business affairs. "It would have to be a lot more than money; it would have to make a ton of sense and be super-strategic. So far, we haven't seen anything like that."

When Russakoff joined in 2005, Ultra was barely holding on, "a one-day show with reasonably low attendance not doing well financially," he says. Faisbach explains that other festivals had come into Miami and tried to put Ultra out of business, "going into the same venue one week before and paying triple for the same artists."

Native New Yorker Russakoff moved to Miami to focus on branding and the overall experience. Within two years Ultra extended to two days, and set a Miami city attendance record in 2008 with 70,000 tickets sold (and welcomed Heineken, its first major sponsor).

The decision to double the length and size of the festival this year was tied to demand, Russakoff says, and a desire to mark its 15th anniversary with something special. "We are limited in capacity [at Miami's Bayfront Park] and wanted to send a bigger message. We sold out really early last year and knew there was a lot of demand, and from a programming perspective we wanted to do things differently."

Both admitted "festival junkies"--they visit as many festivals worldwide as they can each year--Russakoff and Faisbach set out to change the two-weekend model. "Coachella is Coachella," Faisbach says, "but we didn't want to do what they were doing."

They created what Russakoff calls a "major play" and an "under play": a chance for the same artist to play both weekends but under different conditions. For example, Tiësto will headline the main stage with his pyro show, but will also play prior in the MegaStructure--a big tent, but still notably smaller than the main stage.

"It brings us back to our roots," Russakoff says. "I love the energy of a massive tent. When you're in that environment, connecting with the crowd and the music, it's an experience unlike any other--bigger than most clubs, but less about the production than a main stage."

Ultra is committed, Faisbach says--financially and otherwise--to improving across all facets every year. "We've taken everything and reinvested. We've never been cheap or cut corners. We keep putting everything back in to build the brand. It's not a conventional business, but it's the business I chose. Fifteen years later, it's worked."