Zooming down an 800-foot hill in the darkness, guided only by the blinking lights taped to their handlebars, they are a swarm of red reflectors buzzing around each corner and over every bump. Fifty strong, they whiz past the three-story mansions of Southwest Portland, a band of mini-bike maniacs barreling down the wide deserted streets at 30 miles per hour.

On any given Sunday, up to 60 of them rendezvous at Rocco's before boarding the MAX at Southwest 10th and Morrison and getting a lift up to the Zoo. There, at the top of Washington Park, they admire the lit-up view of downtown Portland at night. Although they have several different routes, their high-velocity endeavor usually begins on the corner of Fairview and Kingston and ends as they coast back into town via Burnside or Salmon.

Zach Archibald rides in front of the pack. By day, the 26-year-old folds bundles of cloth at Jo-Ann Fabrics. By night he is an avid Zoobomber. "It's about riding a bike down a hill, fast, and it makes me smile," says Archibald, a.k.a. "the Hundredth Monkey."

For some, zoobombing is a sport, a weekly adrenaline rush. For most, it's a social gathering, something different from the mundane bar scene. "My mom gets together every Wednesday and plays Pictionary with her friends," says Billee Golden, 28. "We get together and ride our bikes down a hill."

Unfortunately, local law enforcement considers zoobombing a hazard. Zoobombers have become so accustomed to being waylaid by the police they have developed code names for them. Cops are known as "pepper." A police car with lights and sirens is "hot pepper."

WW joined the Zoobombers three weeks ago. Our outing got off to a poor start. As the MAX pulled into the Washington Park station, a troop of "Tri-Men" (TriMet cops) lay in wait, face after stoic face lining the platform.

The Tri-Men slapped 31 Zoobombers (including your correspondent) with blanket six-month exclusions from all TriMet transportation, including both MAX and buses, on the grounds that they didn't stow their bikes in the vertical racks provided.

The Zoobombers pointed out that there weren't enough racks to accommodate them all, that there were no signs stating the maximum number of bikes permitted, and that there was no one else on the train. But these observations made little headway with the Tri-Men.

"Do you know how many laws are out there that aren't written down?" asked one officer.

TriMet officials sang a different tune when they learned that WW had been along for the ride, however, rescinding all 31 exclusions and issuing a letter of apology. "From this point forward, we want to make sure we're focused more on education," says Tim Garling, a field-operations manager for TriMet. "That night, we got in enforcement mode instead of education mode."

"Our goal is to have fun," says bomber Sarah Blount, 24, "but we're walking a thin line. We have to abide by all laws, and we're trying our hardest, but the TriMet police are trying to get us in trouble. For them, bike riders are trouble, unless they are 35 years old, wearing expensive spandex and commuting to work. I don't know if they are expecting to shut us down, but there is no way we are going to stop. We're not going to stop having fun."

The Zoobombers' origins stem from an encounter at Rocco's last summer, when Archibald, fresh off the Greyhound from Texas, saw Golden and some friends roll up on mini-bikes. Archibald lifted his shirt, baring the mammoth tattoo on his back ("One Less Car") and the rest is history.

"When we got to the bottom of the hill, I said, 'This is what I was born to do,'" Archibald remembers. "I knew it could grow."

Within a month, there was a core group of 30 Zoobombers. In the past year, as many as 400 people have taken the plunge. Meanwhile, the group has established a headquarters in the middle-class Concordia neighborhood of Northeast Portland, in an innocuous-looking beige house whose back yard is littered with hundreds of Huffy, Schwinn and generic bikes. Hot pink, turquoise, red bike parts, Minnie Mouse and Barbie decals, bikes made out of wheelchairs, bike parts hanging from the trees: everything one would need to put together the ideal mini-bike.

The bikes are available to anyone interested in zoobombing (mini-bikes are preferred because they're easier to maneuver into MAX cars.) The group also offers costumes, including ski-racing suits, sailor outfits and even a pumpkin get-up.

Zoobombing is not for the faint of heart. There have been numerous skin burns, bloody knees and, once, a broken collarbone (the worst accident thus far). The Zoobombers make frequent stops to make sure no one is left behind and encourage newcomers to abide by all traffic and TriMet rules.

Together with Shift, a grassroots bike organization, the Zoobombers are holding "Mini-Bike Summer," a 17-day event involving camping trips, cocktails, a bike-mobile art party and, of course, zoobombing.

For more information on the Zoobombers, check out www.zoobomb.org .