How could an expedition within one of Maine's cities begin to compare with, for example, watching a sunrise from Katahdin's Knife Edge or a weeklong paddle down the Allagash?

In terms of tasting the full power and beauty of the Maine outdoors, the answer, of course, is it can't.

But perhaps the comparison is unfair. One doesn't often decide between an after-work hike in Portland or kayak camping in Penobscot Bay, between a Saturday morning bike ride through Bangor's City Forest or climbing one of the state's 4,000-footers.

Urban outings are in a category of their own. They are accessible adventures along the boundary between the city and its semi-wild surroundings. And for that reason, these outings sometimes offer insight into the outdoors that longer, more ambitious expeditions cannot.

Just north of Bangor, thousands of nearby city dwellers revisit a square-mile bog to watch the succession of plants and wildflowers throughout the season: from cinnamon fern to sensitive fern, from bogbean to blue flag iris. Who else but a park ranger has seen anything like this unfold on the craggy peak of Mount Katahdin?

In the past few weeks, two reporters set out looking for outdoor adventures in Maine's three biggest cities: Portland, Bangor and Lewiston-Auburn. These expeditions took us around Portland by canoe, through an abandoned gemstone mine in Auburn, across a bog outside of Bangor, and other special places.

We only began to tap the possibilities within these city limits; others might have used the opportunity to seek out urban swimming holes, scuba dive off Peaks Island or hike in Bangor's Prentiss Woods.

Whatever the activity, the most valuable part of getting outside in the city may be the profound shifts of perspective that come with stepping out of a car and criss-crossing the edges between woods and concrete.

The owl that flies over your canoe while paddling the Stroudwater River becomes something more when, 10 minutes earlier, you were portaging through the streets of Portland to reach the waterway. Likewise, a concrete overpass on the Maine Turnpike that you may have driven over hundreds of times might seem like something entirely new when you round a bend while running in the woods and suddenly find yourself staring up at it from below.

Jo Eaton, who for a time ran a business renting kayaks from the Bangor waterfront, recalled such an experience while paddling with a group one day on the Penobscot River just upstream from the city a few years ago. It was a breezy August day with sunlight glinting off the river, and downstream the National Folk Festival was in full swing.

The kayakers were nearing the city but still surrounded by woods when they heard a sound that made them sit up straight and lay down their paddles. Drummers from the Penobscot Nation were playing onstage in Bangor and the wind wafted the music upriver.

"The consensus was everybody should have this experience," Eaton said.

For some, these sort of encounters might lead to ruminations on whether nature is as far removed from our urban lives as we imagine. Others may be content to explore these boundaries more physically, even finding ways to treat the city as if it were another form of wilderness when getting out to bike, run or paddle around town.

Rage on Portland, a mountain biking club, takes this approach a few times a month on an "Urban Assault Ride" through the city. Starting from Payson Park, a dozen or more bikers make a mostly off-road circuit around Portland. Part of their loop follows a challenging narrow trail across jagged bedrock in back of Evergreen Cemetery, tops a boulder and pops out in a residential neighborhood near Hall School. Other sections run along the spine of an old pipeline and underneath power lines.

When they re-enter downtown Portland on outer Congress Street, the bicyclists treat the concrete as just another obstacle on their trail. They swoop up an embankment beneath a highway overpass and rattle down sets of stairs near One City Center.

The urban environment can be at least as unforgiving as the wild one, though. On a recent ride, one member of the group, Matt Rousseau, went to jump a curb and ran smack into a mail box.

"I'd rather fall off some rocks and roots," Rousseau said after the ride. "I'd prefer mud to tar."

Staff Writer Seth Harkness can be contacted at 282-8225 or at: sharkness@pressherald.com