"Zoo-zoo?" A grungy, bespectacled young man to my left shouts across the train platform. A cyclist, rolling idly down the street on what is perhaps the smallest bike I've ever seen, takes notice. "A-zoo-Bomb!" The youth next to me concludes, and the two of them wave to each other.

"You going to the pile?" The first inquires.

"Yeah, I'm gonna hang there for a bit, and I'll be up for the first run," The cyclist drawls.

"Peace," he salutes, and turns to the bemused girl between us. She's adjusting her glasses as if ready to ask, but isn't sure what kind of weird answer she'd get back. "Any time you see someone on a tiny kids bike on a Sunday night," he explains cheerfully, "Chances are they're going on Zoobomb."

"Zoobomb? What's that?" She asks, now alive with curiosity.

"Basically, a bunch of us take, like, every kind of bike imaginable, ride on to the MAX, and we get off at the Zoo stop, you know? Yeah, like 500 feet underground. Then, we cram ourselves into those elevators, and once we're at the surface, climb all the way to the top of the hill. Then, at about 10:30, we all ride down, like 100 bikers in a massive pack, craziest thing you've ever seen in your life. We blast down that hill at 30, 40 miles an hour, in the dark, on these tiny little bikes. It's unbelievable."

"So," She pauses, uncertain. "What's it like? To go down the hill, I mean."

"Life changing," he grins, and hops on his bike. "You gotta try it someday!" She looks intrigued, but has a train to catch. I, on the other hand, know I've found exactly what I'm looking for. Four years ago, my swashbuckling friend Ryan told me that I needed to go on this crazy bike ride myself. I turned him down at the time, but now I'm here to try it at last. Jumping on my own bike, I follow the first rider down to the Pile.

It's both unconventional sculpture and flagrant abuse of an perfectly innocent bike rack. The Pile lives up to its name: roughly 20 kid-sized bicycles lashed ramshackle to a single steel bar. As if on cue, a throng of Portlanders rise from the Rocco's Pizza across the street, and wheel their bikes to the Pile. Over the next half hour, dozens of cyclists arrive, as colorful and unique as their unusual vehicles: minibikes, BMX bikes, double-high frames, road bikes, and skateboards. Some come with extra wheels for stability when crouched to minimize drag, others sport six foot dragster-like front forks. There are zebra stripes, mismatched wheels, and sparkling tinsel through wheels.

The riders themselves are mostly young, in their teens to twenties. They wear worn jackets and torn T-shirts, show off punk hairstyles, or pierced noses. One boy about my age sports a black underarmour shirt, full-face motorcycle helmet, elbow-pads, and bright blue pants which reveal tough-looking armor plating where the fabric has worn through. There are a few incongruously proper business-types in their forties, but the norm here seems comfortably iconoclastic.

One young man takes charge of the Pile, asking everyone without a bike to step forward. He has them sign off on a clipboard, and distributes emergency kits to those with medical training. The rest of the riders check their bikes, admire the latest modifications and finds from garage sales, and discuss recent cycling events. "Were you there at the naked party? They had me doing handstands on my bike, junk hanging out to here, man! It's like 'Whoa, don't slip up', yo!". A young man (bearing a more than passing resemblance to a hobbit with his cape) casually strips and repairs one of the Pile-bikes in less than five minutes. Troubles resolved, he hands the bike off to a waiting rider, and the call goes out: we're on the move.

I can't help but grin as we proceed up the pavement, a swarm of cyclists two blocks long. We flow across streets, up hills, a liquid which defies the natural downhill teleology. We split apart when traffic intercedes, but pool together once at the MAX stop. Then comes the squeeze: the train arrives, the doors slide apart, and a hundred bicycles with riders are sucked into the light rail car. We're amiably compacted, elbow to spoke, careful to keep greasy tires out of the faces of other commuters. For the most part, those already on the train are unfazed: ride the MAX in Portland, and you'll see this sort of thing on a regular basis.

The train edges out of downtown, and picks up speed as we barrel into the tunnel. We're deep under the West Hills, and the ethereal howl of the tracks permeates the train. After a few minutes, we slow to a halt, and the doors open. Riders spew out of the packed cars into the underground station, waving their thanks to the driver. Again we play the jigsaw puzzle, this time cramming twenty-odd bikes into the giant elevators at the end of the station. It's a good chance to get to know your fellow Zoobomber, as you help them hold their bike overhead. We rise up, counting off the 260 feet, and emerge exuberant into the cool night air.

Now the ride takes on an edge of something secret: without apparent organization or direction, we walk, ride, and hike up the hills around the zoo, arriving at the top just as dusk falls. There's down time: riders talk and drink, and loose smoking circles aggregate. Finally, there's a proclamation: "FIVE MINUTES!" We huddle round for a quick introduction to Zoobombing protocol--how to warn other riders of traffic, the proper way to announce intention to pass, and a description of the route. "If you get lost," our educator slurs, "Just head downhill."

Seems simple enough, and five minutes later, I'm holding onto my bike, surrounded by hundreds of unconventional cyclists, counting down the seconds. "Five! Four! Three! Two! One! Zoobomb!" We shout in unison, and set off down the hill. As we pick up speed, the surrounding riders start to feel like a stationary frame of reference: it is the rushing gray asphalt, lit up by dozens of high-intensity bike and helmet lights, that is moving beneath us.

"Passing on your left!" I hear a shout being me, and maintain space for them to ride through. Seconds later, a woman in her forties on a tough-looking mountain bike blazes past, her broad grin a blur. "Zoobomb!" She shouts, and I echo it back to her. We're yelping, shouting, exuberant as the road bends and twists through the night. Each curve brings an exhilarating burst of speed, as we corner hard into the road.

Up ahead, riders shout out a warning. "Car!" It's a signal that a vehicle is near, and sure enough, just past the next turn an SUV idles. Its lights flare brightly and make it hard to see the road. "Car!" I shout, doing my part to warn those behind us. Most drivers just stop when they encounter a Zoobomb: there isn't any other safe option. Cyclists break and flow around the vehicle like fish around sharp coral, rejoining on the other side with whoops and grins.

At the base of the hill, I see the group slowing to a stop: we've reached the halfway mark, a cemetery drive just off the main road. "WOW!" the curly-haired college student to my left is overwhelmed. "Did you see how fast we were going?"

"Naw, you guys are slowpokes," the second man on a tandem-bike team contradicts. "We made it down here *way* before all of you."

"You're the blind biker? Seriously!?"

"Yep, legally blind. 'Course, my partner here isn't, he steers for both of us. But you get two guys pedaling on a single frame, and nothin's gonna catch you." I want to ask how they do it, but I'm interrupted by the start of the second half.

"Pick whatever direction you want, and go!" A voice shouts out in the dark, and I follow the cyclist ahead of me. "Stay out of the center," is all the warning I get, before we plunge onto a narrow, broken-up, U-shaped path: more a trail, really. The asphalt has ripped and cracked in the center, and my bike shakes violently--before I know it, my light has slipped free and is hanging loose from the handlebars! Following the riders in front of me, I dodge rocks and puddles as we barrel through the woods, finally bursting free into downtown streets.

Luckily, there's nobody behind us, and our stream expands to take over both lanes. Cars ahead of us come to a stop, and we whiz past. At each light the front riders shout the status of the intersection: "Car!" indicates cross traffic, while "Clear!" indicates it's safe to enter. Some runs are timed; the Zoobombers count down from red to determine when to start their approach, and make every light. On this route, there's no way to determine the state of the lights, and I feel lucky to make the greens. We finally coast across the PGE Park triangle, and arrive at the MAX station. Time elapsed: fourteen pulse-shattering, adrenaline-inducing, incredible minutes.

What's next?

We do it again--but this time, up the stakes.

They call it Hellway, but most people know it as Highway 26. Our route goes downhill right from the elevators, and there's no looking back: all eyes are focused on the road ahead. We drift around the Zoo parking lot, cross the freeway overpass, and make a left turn onto the on-ramp. We drop to a crouch, some riders hanging just inches above the frenetic spin of their back wheels, and pedal as hard as we can to get up to speed. The freeway drifts closer to the on-ramp, we fill the lane, and... merge! The freeway margins are host to deadly debris: gravel, glass, and metallic remnants of car accidents which will puncture your tires or convert your face to chiseled spam in the blink of an eye, so we take over the right lane at forty miles an hour.

Cars drift by in the leftmost lane: there's next to no-one on the road, but what drivers there are give us plenty of clearance. Despite the steep downward grade, I have to pedal as hard as I can to keep up: daredevils on minibikes draft in my massive air shadow, and then zoom past, tiny wheels whirring. After a couple of miles, we take the off-ramp, and power through the tunnel under the West Hills. It's an awe-inspiring sight: fifty-odd bikes flying beneath the harsh sodium lamps and arched tile walls. We burst out into the free air again, and, coming into the final stretch, coast under the soaring central span of the Vista Bridge. It's fantastic, awe-inspiring, like nothing I've ever done. When the train comes, ten minutes later, we're still grinning ear-to-ear.

Zoobomb: one heck of a bike ride.