Mark St. Amant was at a cross-roads. The 36-year old Associate Creative Director at Keiler & Company, an ad agency in Hartford, Connecticut, was finding it difficult to balance his day-to-day work responsibilities with his growing obsession with fantasy football. Something had to be sacrificed. So like any ambitious urban professional with a healthy sense of priorities, he made the most logical decision.

He quit his job.

“I’d been playing in an office league since the late 90s and never won,” he explains. “I came close a few times, but it was an always-a-bridesmaid kind of thing. I just got to a point where I realized, I think this job is really draining my time and preventing me from winning this thing.”

Amant admits that, in hindsight, it maybe wasn’t the best career move. Giving up a six-figure-a-year salary for the chance to win a $700 first place prize in an office league is not exactly an air-tight financial plan. But for him, it was never about the money. “It was more about the bragging rights,” he says.

Welcome to the frequently nonsensical but always passionate world of fantasy football.

For those unfamiliar with the sport, fantasy football is a group competition in which participants (called “owners”) create their own fictional NFL teams, drafting actual players and scoring points based on the statistics of those players. To outsiders, it may sound reminiscent of another classic nerd pastime, Dungeons and Dragons. And the comparison, as even hardcore football fantasy enthusiasts will admit, is not unfair. The only real difference is that instead of mythical creatures like basilisks and jackalweres, the characters in fantasy football are real and have names like Tom Brady and Aaron Rodgers.

Before Amant became a convert, he was like any other office worker who’d never heard of fantasy football. “I thought it was a quasi-nerdy hobby,” he says. He remembers the way co-workers would look at the fantasy football people, who gathered in conference rooms to discuss their latest trades. “They’d just roll their eyes, like they were thinking, ‘Ooooh, you fucking dorks.’ And I kind of agreed with that.”

But his opinion changed in 1998, when he was invited to join a league at his office. “Somebody had dropped out at the last minute and they needed a twelfth for their league,” he says. “I happened to be there at the right time. They were like, ‘Do you have $50?’ I was cynical about it. I was like, ‘Yeah, whatever.’ I thought they just wanted me for the money. But a few weeks later, I was hooked.”

Although quitting his job to focus on fantasy football didn’t pay off exactly as he’d hoped—even with all that free time, he still didn’t win his league—it did work out for him in the long run. He’s currently employed by Sterling Rice Group, a small ad agency in Boulder, Colorado that actually encourages his fantasy sports infatuation. (One of his first projects for the agency is a fantasy football tie-in/promotion for the California Almond Board.) And during his day-job hiatus, which lasted for almost three years, he blogged about fantasy football for the New York Times and made frequent appearances on ESPN as a “fantasy football expert.” Sometimes, like on the now-cancelled ESPN show Classic Now, he got into heated debates with Meat Loaf, the portly singer best known for classic rock hits like “Paradise by the Dashboard Light.”

“Meat Loaf became my nemesis,” he says. “At the time, Meat was involved in something like fifty different leagues. He’s really into it. But it was bizarre to argue with him. I’d be like, ‘I lost my virginity in a car while a Meat Loaf song was playing. And now I’m arguing with him about Peyton Manning versus Drew Brees.’ It was just very surreal.”

His latest fantasy football nemesis is Shergul Arshad, who he describes as a “nefarious Pakistani/Italian guy who always wins. He’s the devil. He’s always had these nondescript venture capital startup jobs that allow him to travel around the country and visit NFL training camps, where he tries to weasel information from the players. He takes it to a pretty extreme level. He’s more obsessive about it than I am.”

Amant thinks about that last statement for a second before reconsidering. “Well, almost,” he says.

Amant and Arshad are hardly in the minority. According to to the Fantasy Sports Trade Association, there are 30 million fantasy players in the United States and Canada, an astonishing 54% increase from just two years ago. And with an average income of$81,000, it’s no surprise that most of them also work full-time at offices. At least judging by the numbers, it would seem that this once maligned extracurricular activity has officially become an accepted mainstream phenomenon.

That assumption would be very, very wrong.

Late last year, four employees at Fidelity Investments in Westlake, Texas, were fired for alleged participation in a fantasy football league, in violation of the company’s gambling policy. Among the accused was 26-year-old Cameron Pettigrew, who worked as an account representative at Fidelity for almost three years. After two instant messages referencing football were discovered on his computer, he was taken into a conference room and grilled about his fantasy football activities for, by Pettigrew’s estimate, 90 minutes. “They interrogated me as though I was some sort of international gambling kingpin,” he told reporters later.

It was, as Pettigrew has repeatedly pointed out, an awful lot of paranoia surrounding an office fantasy football league with a $20 buy-in.

While Pettigrew’s case may’ve been an overreaction, CEOs and managers do have some justification to be paranoid. Challenger Gray & Christmas, an outplacement firm in Chicago, estimates that because of fantasy football leagues, many companies could be losing as much as $1.5 billion during an average football season, and all because league members are spending as little as ten minutes a day working on their fantasy football rosters. With that kind of lost income, it’s a wonder that workplace fantasy football hasn’t spawned it’s own version of the McCarthy hearings.

So what happens when one of the most popular recreational activities among office workers becomes a source of controversy and job insecurity in an already uncertain economic climate? If you think office workers have been giving up their fantasy leagues, you obviously don’t know football fans. Rather than disappearing, the football fantasy leagues are just going underground.

Kevin Akansky, a 38-year-old senior director of marketing for a high-tech software company in the Washington D.C. area, compares today’s fantasy football leagues to Fight Club, the 1999 movie (based on a novel by Chuck Palahniuk) about a secret cult where guys meet in basements and get into violent bare-fisted fights. Fantasy football may not be as sinister, but it’s definitely as clandestine.

“It’s become like a secret fraternity,” Akansky says. “You can talk about it, but don’t talk about it publicly. Because it’s not perceived as professional. If you want to move up the corporate ladder, your fantasy football allegiances have to become more hush hush.”

He was once told by a senior executive at his company that any involvement in an office league could damage his reputation. “He actually asked me to stop playing,” Akansky says. “I didn’t, but I certainly kept it more low key. Whenever I talk about it now, I’m always looking over my shoulder to see who’s listening.”

As fantasy football has become increasingly surreptitious, it’s also gotten weirder. Amant, who wrote a book called Committed: Confessions of a Fantasy Football Junkie, has heard tales of fantasy football leagues that sound like urban legends. In one workplace league, he says, the loser is required to sprint naked through the office, painted in the colors of the winning team. In another, run by co-workers at an undisclosed business in New York City, the draft pick running order is determined by a semi-regulated boxing match.

“They rent a beach house on the Jersey shore and make a weekend of it,” he says. “They bring their families and everything. And then they beat the living shit out of each other to decide who gets the first draft pick, and then the second and third and so on. It must be strange when they show up at the office on Monday morning with a black eye or broken nose. Their bosses are probably like, ‘What the hell happened to you?’”

Broken noses and secret meetings? Why does that sound so familiar? “It’s actually kind of like Fight Club,” Amant admits.

Amant attempted to put us in contact with several participants in these unorthodox office fantasy leagues. But despite offering to keep their identities anonymous, they declined to speak with us on the record. The general consensus, in Amant’s words, was “I’d rather not disclose my sick hobbies for fear of getting canned.”

“What’s that great quote from Fight Club?” Amant laughs. “‘The first rule of fight club is you don’t talk about fight club.’ It’s the same for some of these leagues. The first rule of fantasy football is you don’t talk about fantasy football.”

Nervous denial became a recurring theme among many of the people we interviewed for this story. Take, for instance, “Ronald” (not his real name), who works for a law firm in Chicago and has been involved in fantasy football leagues for the last ten years, even developing an elaborate 10-sports league that runs year-round. At first, he downplayed the dangers of being involved with fantasy leagues at the workplace. “I don’t talk to my employers much about it,” he said. “But I’m certainly not afraid to.” He then asked that we not use his real name or include any details about his various fantasy leagues.

“It was easier to talk about this stuff back in 2001, 2002, when it was just a nerd fringe thing,” he says. “But as it’s became more mainstream, and companies started developing internal policies regarding fantasy sports, you’ve got to be very careful about what you say.”

Dustin Ashby, the commissioner of the World Championship of Fantasy Football— a high-stakes league that holds its main event in Las Vegas, with a $300,000 grand prize — thinks fantasy sports have an unfair reputation as a drain on office productivity. “It’s a healthy game,” he says. “It’s not illegal or negative to society. It’s harmless. Fantasy sports are the new water cooler topic. It bridges the gap from top-level executives to mail-room clerks. And that creates social dialogue and breaks down barriers of communication. It really does create a community in the workplace.”

A workplace community that, according to some, has the intellectual sophistication of prepubescent boys. “When we get to trash talking, the humor is on a third grade level or worse,” says Kevin Akansky. “But that immaturity is part of the fun. It’s a chance for people who normally have to be completely professional to unwind and drop all kinds of f-bombs around each other. You get to be a teenager again and do things that should never, ever happen in the work place. Fantasy football is a release.”

While fantasy football can turn people into idiots, it can also make them highly motivated and hyper-productive. Kyle, an IT manager for a telecommunications company in Overland Park, Kansas, says that his co-workers involved in fantasy football are sometimes the first ones to arrive at work in the morning. “I remember one week there were a lot of injuries (in the NFL) and a few players had some really monster games,” he says. “So the next morning, there were like two dozen guys in the office around 6:45 AM, working at their desks and making new drafts for their fantasy teams. The entire office, for like an hour and a half, was all fantasy football.”

Some office workers, like “Glen Patinkin” (not his real name) in Brooklyn, New York, take a slightly more dangerous approach. He and a fellow co-worker are involved in an online league, with eighteen members across the East Coast. On draft days, they usually visit a bar in the city, where they become “way too drunk to pay attention to what we’re supposed to be doing.” This year, however, they’ve decided to try something different. At some point during this coming Labor Day Weekend, they’ll be “borrowing” the office building where they both work in downtown Manhattan.

“There won’t be any security in the office itself to worry about,” he says. “It’s not against the rules for an employee to be there during off-hours, so the security logs and cameras won’t be an issue. And we’ll be carry-in, carry-out with our provisions, of course.” And what provisions might that be? “A large pizza and a six-pack of beer, which I’ll probably carry in my messenger bag to be unnecessarily discrete, and a misdemeanor that we’ll have smoked en route.” Once inside, they plan to find a conference room big enough for them to “spread out our papers, laptops, and food,” where they’ll have access to a big screen flat-panel monitor and the ability to set up conference calls with their league-mates.

“In our younger days, this might’ve been just the beginning of a much wilder tale,” he says. “I’d be sneaking in strippers hidden in a pantomime horse outfit. But I think the basic narrative here is: two guys escape their adoring families to catch a buzz and geek out on some high-definition imaginary football.”

A sobering thought for any CEOs or senior executives hoping to enjoy a worry-free Labor Day. As you prepare for a holiday picnic or barbecue, take a moment and wonder, do you know what your employees are doing this weekend? Or more disturbingly, where they’re doing it? Next Tuesday, if your conference room smells like stale beer and face paint, well, now you know why.

(This story originally appeared, in a slightly different form, in the September 13, 2010 issue of Bloomberg Businessweek.)