It’s 2 A.M. and T.C. O’Rourke is leading a handful of bikers sweating and swearing through the heavy gravel of an old rail bed. They’re on an "Urban Assault," a late night tour of Chicago’s wasted parts. The riders crash through industrial lots and rail yards, over crushed pavement, past heaps of metal scrap. Then they summit some wall, bridge, or roof- top and look over the city. Twinkling beneath them, Chicago calls out from a thousand unvisited corners. "You can’t justify it; it’s trespassing" O’Rourke admits. "Then you’ll glance down some dark alley and think, ‘I wonder if I can go through there, I wonder where I’d come out.’"

The first urban assault was held five years back, when the guys at the Rapid Transit Cycle shop wanted to show the visiting editors of Dirt Rag magazine a good time. Rapid Transit’s owner, Chris Stodder, says his guests were unmoved by the area’s regular off-road options, "but when I offered to take them through the underside of the city, they perked up." Stodder showed them the routs he liked to ride with his friends: shortcuts they had used as bike messengers that they sometimes revisited at night.

Now the urban assault is a semi-regular, semi-organized event. Anyone can pick a rout and announce a ride; news goes out by word of mouth. The typical crew is a bunch of guys in their 20s, joined by the occasional female. Jim Redd, who’s past 50 and programs software by day, instigates some of the wildest rides. He favors those parts called the "Gothic estuary," a maze of streets and rails that runs below grade down by the Chicago River. In the past he liked to emerge in the vast, old rail yard south of the Loop, overgrown with native wilds. Crawling out from the dark into that secret prairie, he and his crew would look up to see the skyline glittering above the scrappy trees. Not any more. On the first mild night of 1999, they came back to find fences in place, the ground turned up for new development. "There’s just not enough wasted space anymore," Redd sighs. Some of these places haven’t seen development in more than 30 years; the buildings emptied out as the people drained toward the suburbs. Now the builders have come back, and their taking over the best routs,

Not that redevelopment will put an end to urban assaults: The recently gentrified streets are just a new surface to try with a bike. You can always race up the ramps of parking garages, jolt down flights of stairs, or rouse the bored security guards for a race through the Metra stations. But the best rides, O’Rourke argues, are the ones that make unexpected connections from one part of the city to another. Just because you can see a place in the distance doesn’t mean you can find it. If you do, its often by chance. On certain nights O’Rourke says he’ll come out on top of some mountain of slag, or find himself in a field of gutted cars, and realize, "Wow, this is that place I used to see from the train when I was 12 years old."

"We’re Americans, we need frontier," Michael Burton philosophizes, "a place to strike out, to see what there is." During the summer, Burton leads Sierra Clubs trips out West, but in his urban life he says he finds the promise of the frontier "in the parts of the city coming apart-where the human touch has withdrawn for whatever reason, and nature over takes it. I’m not sure I’d call it wilderness; it’s a frontier of a different kind."