The parking lot at Toronto's Crothers Woods trailhead does not reveal its secret. If you believe the kiosk, a few little trails spider down a hill and around some forested flats, forming a pleasant, if insignificant, network of beginner-friendly singletrack.

When we arrive, at 9:30 on a Thursday morning in October, only two others are there. A friend has arranged for us to meet a local bike mechanic for a tour, and another regular has joined him. By tomorrow morning, after nearly eight hours of pedaling, it will be clear that Toronto holds the most unlikely promised land in North American mountain biking, a bounty of such challenge and character and unfathomable variety that I will struggle to explain it for months. But now, standing across a swath of asphalt from the Loblaws grocery store, I cannot help but think the Don River Valley has been overblown by locals who need to get out more.

Andrew Maemura, our 35-year-old tour guide, who goes by Maki, fidgets off to the side as his childhood friend Matt Morrish, photographer Bruno Long and I don shoes and packs. Paul Stuart, 39, a construction worker who has been riding in Toronto for 27 years, rests on his dirt jumper, sucking on a cigarette. A wallet chain hangs from his jeans.

The trail network, in a pair of glacial ravines known simply as 'the Don,' carries an almost mythical reputation among those who frequent it. With 60 miles of handbuilt singletrack wedged in a city of 6 million people, it's hard to comprehend how it remained a secret for so long. But before Trailforks and Strava, only those who knew someone—or were willing to spend hundreds of hours exploring—had any idea how expansive it was. "It was kind of like 'Fight Club,'" one local told me. "Honestly, in the early days, you did not talk about trail entrances and exits."

Captain’s orders: Matt Morrish takes five at the shop.

We pedal down an official trail that soon gives way to an unofficial ribbon through the foliage, spitting us out at a homemade freestyle playground with dirt jumps and a pumptrack. Stuart, known as Digger to some, basically lives on this dirt. He empties the trash cans, pushes a wheelbarrow, shovels clay and mans the grill. Someone once did 201 laps on the pumptrack without pedaling, he says. "I counted."

Maki waits on the edge while Digger laps the jumps. "As of 2000, this was all vegetated," Maki says. "There was one singletrack through here."

Maki is dressed in a black 'Don Vivants' jersey, fluorescent-yellow arm and leg warmers, and white socks with red polka dots. The son of World War II immigrants—his father is Japanese, his mother Ukrainian—Maki grew up racing cross country and getting lost in the Don. He's worked in bike shops since he was 16, including the last 15 years at Cycle Solutions, where local bankers buy high-end Santa Cruzes and fete their clients in the Don instead of on a golf course. According to longtime builder and outspoken advocate Timothy Charles, aka the Donfather, "No one knows the Don better than Maki."

We continue our ride along the chain-link fence and sewage plant, inhaling a putrid smell that is part human feces and part squirrel carcass hanging on a tree branch at nose level. Digger bails soon after that, deciding his dirt jumper is not the right tool for this mission. Maki, whose quads look like concrete columns, cleans a technical section over ladder bridges and rockrolls before topping out next to a cement factory at a plateau called the 'Doobatorium.' Later, we pass the Trump Jump, or at least its remnants. It was built over 18 months, shortly after another trail had been ripped out, to "Make the Don Great Again." It lasted about a month before city officials apparently decided a 20-foot gap jump onto a narrow, tree-lined runout was not liability-friendly.

"Let's duck down here," Maki says at a junction. "There's a new trail that's really primal." We follow him through an Ewok forest and eventually up a series of the tightest switchbacks I have ever ridden—built out of necessity to maximize the limited land—on a trail called Climbmaxxx. We pass a half-buried shopping cart and lean into berms reinforced by a vehicle tire, logs and concrete slab. At one point I think I glimpse a deceased toaster oven peeking out of the ground, but I can't be sure. The brief descent ends at a giant boulder that thrusts riders toward a canopy of leaves above, named—what else?—the Ejaculator.

Bart Simpson’s maple-leaf modesty overlooks the author and Jeremy Lootsma.

During a pee break, I tell Maki that I recently visited a trail network that was funded by more than $600,000 in grants and donations. "That's funny," he says. "These trails were built on two coolers of beer and a lot of time and sweat."

Eventually Maki leads us to the east fork of the Don River, aka the East Don, where trails remain more rugged than in the Main Don. We ride Kitchen Sink, so named for the kitchen sink that some early regulars found along the trail, causing one to exclaim, "This trail has everything and the kitchen sink!" The warm, forested rim gives way to a bona-fide jungle, then a cool bamboo forest where we weave between stalks along the river. I hear something flapping and glance over to see dozens of 20-pound Chinook salmon spawning in six to eight inches of what looks like glacier water. I feel like I am 100 miles from a city—or 1,000.

"Do people swim in the Don?" I ask Maki.

"I wouldn't recommend it," he replies, and soon I will find out why.

Don patrol.

Rarely does a climb in the Don last longer than 100 vertical feet, which is about how much we punch up to a perch under orange foliage high above a gorgeous bend in the river. The breeze blows and Chinook flap as we look out over a fall forest, high-rise apartments in the distance. When I booked my trip to ride in Canada's biggest city, I did not expect to be physically worked and gaping at the natural landscape, but suddenly I am.

"This is my favorite place in the Don," Maki says, breaking the silence. It's easy to see why.

We pedal on to Motown, a noodle of dirt that snakes through a jungle of dog-strangling vine. A visionary named 'Quiet Steve' built this trail over a period of three years. We can see the CN Building—the famous needle in Toronto's sky—through the trees. It occurs to me that we have encountered exactly one person since we entered the East Don two hours ago, despite bluebird weather and estimates of 10,000 or more who ride the Main Don.

"You'd never know it," Maki says, "but a few hundred feet from here, six lanes of cars are sitting in traffic."

Before it became a riding hot spot, the Don was a dump. People threw their trash down ravines, where a large homeless community remained for decades. The river is cleaner now, but at one point it ranked among the most polluted waterways in North America. Big flood events still cause sewage outflow, wherein raw sewage washes into the river and Lake Ontario a handful of times a year. But usually the water is at least clear.

As the ravines cleaned, old social trails lured riders deeper into the Don—which is named after the River Don in Yorkshire, England, itself named for a goddess of Celtic mythology. Other riders built new trails, eventually creating a network that attracts everything from 160-millimeter forks to carbon hardtails. Some of the most expensive houses in Toronto back to the Don because of its recreational access. Locals think it's righteous that $12-million castles share a backyard with low-income apartment buildings.

Matt Morish arcs past old car tires, the former trash receptacle now boasts berms.

The Don is home to a handful of outdoor organizations, including the Wildbettys, a women's mountain-biking club with 132 members and IMBA-certified ride leaders. The club's official motto is: "We do it in the woods," but you might also see: "There's no better place to do it than in the Don." While casting a wide net for sources, I heard about another group called the Don Vivants and exchanged a few emails with a member named Ben Aylsworth, who also happened to be one of the core trailbuilders. I was trying to figure out what, exactly, the Don Vivants were.

"This is a gang," Aylsworth wrote. "And I mean gang. We are anti everything. Intentionally leaderless. Intentionally off the grid. But also pro everything. And we love the Don."

I was pretty sure they wouldn't kill us if we rode with them, but Aylsworth said they weren't sure they wanted to let us in. After a group discussion, they decided it was OK and Aylsworth apologized for his gruff first impression. "I'm just protective of this little slice of urban heaven. What I do in the Don really defines me and I think that's true for most of us."

Aylsworth gradually opened up and became a trusted source. A 45-year-old media executive and father, he chased first descents in a kayak for years before finding the Don and deciding it had what he needed, too, but was closer to home.

"My job is high stress, giant events, logistics, and I need a place where I can let that go, not be judged and do whatever the hell I want to do," he explained. "I think that's what the Don Vivants are for a lot of people. We're in the heart of the fourth largest city in North America, but nobody knows we're here. We can kind of just let loose and run around wild in the Don. That's important for my life balance."

I found it ironic to learn that he and other locals sometimes feel insecure about their province's 'Onterrible' reputation as a riding destination, given what exists in the Don. But those who know it, revere it. As Maki said: "I'm a simple person. This sounds weird, but someone once asked me, what are your life goals? Well, I remember thinking when I was 17 years old—still in the hardtail, V-brake era—I just want to be so good at riding the Don that people know who I am just because I ride the Don."

Since the Don is mostly owned by the city, which is short on resources like every city in North America, it sees almost no public maintenance. But local builders say it is not uncommon for trail users—especially runners—to give them money for materials and tools. One builder, who wished to remain anonymous, said a female trail runner once jogged to the ATM, withdrew $150, ran back to hand him the cash, then continued on her way. Another sent $200 to him via email. He has no idea who they were, but he tracks how the money is spent on a spreadsheet.

Left: Sleeves: optional. Right: The Wildbettys proudly call the Don home.

The Don is not the only place to ride in Toronto. Trails and culture abound in ravines and open space across the city, like at the renowned Sunnyside Bike Park and in Etobicoke Creek, where rabbits dart across waterfront singletrack and graffiti under the Queen Elizabeth Expressway reminds everyone to "Fuck 'em. Do you." Some in western Toronto think the Don gets too much credit, but there is no denying it's the epicenter. This is also why it remains a delicate topic in interviews. Locals have been trying for years to move past their don't-ask, don't-tell standing with the city, since they fear deep down that the trails could one day be shut down or dumbed down—or they could get in trouble for building more.

But their efforts have always been stifled by bureaucracy, as well as a lack of mountain-biking knowledge or interest at City Hall. Lately there is new blood and renewed optimism, however. Ian Girard, 34, the inventory manager at Sweet Pete's bike shop and a Don Vivant, is one of six people involved with launching the Don Valley Trails Association. Its mandate is still "loosely defined," Girard says, "but basically it's to preserve, maintain and, I want to say expand, but it's really not an expansion because it's already there. It's more to preserve it."

To find out where the government stands, I met city environmental specialists Karen Sun and Mike Halferty one afternoon at Crothers Woods. Each grew up in Toronto but had no idea the Don contained such a bounty of trails—in addition to beaver, otters, mink, deer and coyotes.

Left: Campy Camp is all about good times for the Don Vivants. Right: 12 people at $100 each = the world’s largest portable speaker.

When asked whether the city is interested in legitimizing the network, Halferty said yes. "The usage is there. The justification is there," he said.  Sun, standing next to him, added: "We would love for our rec staff to be running mountain-biking clinics on these trails. They haven't because they don't know how to. You have to understand, mountain biking is cutting edge in the eyes of the city. Our parks staff, if they can't drive a truck to it, they're not really interested in going in there."

Nevertheless, city officials often find themselves caught between Don trail users and railroad workers, who run trains through the Don. "We recently got a call from Metrolinx asking if we have any contacts in the mountain-biking community, because 'they keep cutting down our fences,'" Sun said. "And before that, a local mountain biker asked me if I had any Metrolinx contacts because 'they keep putting up fences.'"

Five years ago, the city drafted a Natural Environment Trails Strategy to address the estimated 240 miles of trail on its land that it didn't build and doesn't manage. "I think what it comes down to," Halferty said, "is the city's liable for anything that happens on their land, regardless of whether it's sanctioned. So we might as well have good trails rather than shitty trails."

To that end, when I asked Aylsworth why he was OK being identified in this story as a builder, given that the practice is technically illegal, he said: "I don't know. Maybe it's a mistake. But I also feel like the city knows that we're here and they know what we're doing. And we're getting to that point where maybe we shouldn't pretend that this isn't happening and we shouldn't pretend who's involved."

It did not seem like change was imminent last fall, but the dynamic had also come a long way from yesteryear. "The city moves slowly because it's big," Sun said. "You don't want things to move too fast. If the glaciers move too fast, everyone dies."

"But once they get rolling," Halferty said.

"Yes," Sun agreed. "Once they get moving, big things get done."

It could be a serene autumn setting in a remote, quiet forest but no, Morrish corners casually within a city of 6 million.

It is hard to imagine the Don as a legitimate riding zone, with trail signs and kiosks and rules. It almost feels sacrosanct, like putting high rises in Yellowstone. But until some sort of movement occurs with the city, everyone tries not to think of it and simply rides.

We meet the Vivants just before dusk behind Loblaws. I'm still beat from riding five hours with Maki this morning, but it quickly becomes apparent this will not be a hammerfest, as some of their Thursday-night rides are known to become. The crew sizes us up like fish in a bowl. There is the bearded, red-tank-top-wearing Bosnian Alen Zukanovic, 39, who immigrated to Toronto in 1995 to escape the civil war in Sarajevo; he came up with the name Don Vivants. I also meet Mikester, the oldest rider here at 62, who refuses to give his last name and describes the Vivants as "a beer-drinking, pot-smoking club with a cycling habit, with doses of anarchism, Satanism, and a big dash of community service."

We set off down the hill en masse, following Aylsworth and a few others to the flats and eventually the Doobatorium. We ride over an ironing board on our way to Garbage Cow, a faint trail through the woods that everyone loses at least once. At the top of a particularly punchy switchback, Zukanovic pulls down his chamois and shakes his pale cheeks in everyone's face, yelling, "You got it! Yeah! Yeah! Yeah!"

Left: When life gives you a dump, build trails. Right: Andrew “Maki” Maemura and Paul “Digger” Stuart sneak past Chinook salmon in the Don River, once one of North America’s most-polluted riverways.

After a couple of hours in the forest, we head to a local brewery, grab a few cans of beer, and continue to a bonfire down by the river, at a site I have only heard described as "the Rocket" (I will learn later that it's also known as the Stunt Camp, or Camp Campy Camp, formerly home to 80 feet of skinnies and a 6-foot drop called the 'Donaconda'). When we pedal out of the bush and into the clearing, I behold a missile-shaped projectile that is surrounded by Vivants and belching orange embers toward the sky.

Aylsworth and Zukanovic are hanging out next to the world's largest wireless speaker, which 12 of them bought for $1,200. CupcakKe's "Spider-Man Dick," a raunchy hip-hop number, blasts through the clearing. Right after they bought the speaker, Aylsworth says, they held a '90s rave in the Don and only announced it on Instagram. A bunch of Vivants DJed sets in different locations, which were supposed to build up toward a professional DJ's climactic set. But he crashed on his way to the Rocket and broke his thumb. The Vivants also do a Halloween ride and the annual Don-a-thong, which amounts to 20 guys flying through the forest in banana hammocks.

"We're all just children trapped in men's bodies," Aylsworth says at the fire.

To which Zukanovic adds, "We're looking for something that's missing in our everyday lives. This is why, if any of our friends or family say, 'Are you busy on Thursday night?' I have to say, 'I'm sorry, but Thursday nights are out. Unless it's raining. Then we can see.'"

Toronto gets it.

I float around the gathering, chatting up various Vivants. Among them: Michael Nazwaski, the youngest Vivant at 17, who has to be home by midnight. 'Naz' has been keeping up with Maki in the Don, which qualifies as a big deal in this circle, though Maki still treats him like a little brother. Mikester sidles up at one point and says, "A lot of the builders are here. These guys are gods. Honestly, I would do anything for them."

Before long someone introduces me to Timothy Charles, and the Donfather and I back away from the fire to talk. He is dressed in a plaid flannel shirt and jeans, which is about as formal as he gets. When I'd asked the Vivants why they call Charles the Donfather, one of them joked, "Because he likes it." I can see that as we talk, but his kingpin role is also evident. He started riding dirt bikes in the Don in the '70s and first pedaled it in 1981. "I'm the original bad boy down here, dude," he says. "You can mention my name anywhere you want. But I'm just a citizen."

Around 2005, the Donfather, who is 54 and works as a general contractor, formed an informal race team called the Donrats, basically the precursor to the Vivants. For nine years, he never missed a Tuesday- or Thursday-night ride. He'd grown up rowdy, once jumping 13 kids off a plywood ramp in the schoolyard, and spent part of his adolescence in Santa Cruz, where he learned the meaning of flow. "Just imagine yourself inside a giant teacup, and you're just living in there," he says. "That's been one of the greatest joys of my life."

Alternative transportation.

A trio of newcomers emerges from the bushes, and the Donfather ushers them into the circle. "Hello, hello, welcome!" he says.

The Rocket, which is actually an old metal duct that they converted to a wheelbarrow to use while building trail, then abandoned at Camp Campy Camp to anchor their parties, lights up the night every few seconds. The Donfather goes on.

"My vision of what a trail should be," he says, "is you should be able to ride it without having to pedal. I realize there are certain realities with that; we're not in Whistler. But that's the concept. And I love to fly, OK? Humans are fightin', flyin' or fuckin', let's face it. I fly, baby."

The party continues until 1 a.m., when only four of us remain. The Rocket has long since simmered to a faint orange glow. Digger and the Donfather are ready to go home, so we all trudge through the bush and out of the valley where the secret remains.