It's not easy, being a skateboarder in Boston. The urban landscape might be a hard-edged heaven of granite and cement, but the skaters can tell they aren't welcome: Neighbors glare, businesses set up obstacles, and police constantly shoo them from the Copley. So it's revolutionary, in a way, the news that has been spreading through skate shops and the city's forbidden plazas. At the northern edge of the Central Artery Tunnel, in the shadow of the Leonard P. Zakim Bunker Hill Bridge, the state is offering a rare urban gift, a 40,000-square-foot concrete skate park, one of the largest in the country.

The plans being circulated are so vast, so cool, that teenage boys in baggy pants are murmuring their highest form of praise. This park, they say, is going to be “sick.”

Perhaps it’s not the most Bostonian idea, that one of the Big Dig’s most anticipated features will be largely designed and used by 14-year-old boys. But the skate park is for real, a signature project of the Charles River Conservancy, an advocacy group dedicated to maintaining the land along the river, including the new parkland to be developed as part of the highway project.

The group’s leaders have spent the past two years making a deal with the state -- you provide the land beneath the Tobin Bridge onramp, and we’ll build the park -- and quietly pulling public officials on board. They’ve drawn and redrawn plans, and collected about $200,000 toward the $1.1 million they need by fall for construction to move forward for a 2006 opening.

It hasn’t always been an easy sell, said Renata von Tscharner, the conservancy’s founder and president, who said the idea stemmed from a landscape architecture class she taught at Radcliffe in 1998 and from the needs of her own teenage, skateboarding son.

"People said: ‘Conservancy. Aren't you supposed to be about trees?’ von Tscharner said at a fund-raiser Sunday on Beacon Hill. “What are you doing with teenaged boys? What are you doing with concrete?”

But the fact that these days, he’s heading over to the skatepark alone to practice indicates his affinity for the sport has gone beyond father-son- bonding time. “I can go out and I can fall down and make my mistakes and build skills a little bit,” says Malmberg.

But support eventually came, she said, from politicians who liked the idea of a hangout for youths that won’t involve chipped Financial District marble and from Boston police who insist they don't enjoy being spoilsports.

“We just get perceived as being . . . these guys in blue,” said Sergeant Tom Lema, community service supervisor for the downtown district. With a skate park, he said, “we can say, ‘People, I have a place for you guys to go now.’ . . . We can come across as sort of a good person.”

At the Sunday afternoon fund-raiser, a group of teenagers huddled around an armchair nodded in approval at some of the proposed features: It will be accessible by the T, in East Cambridge near the Museum of Science. It will be well-lit and probably open 24 hours a day. One ramp will rise 20 feet in the air. Some of the ledges, designed with the help of Boston skaters, will mimic the Boston Medical Center “hospital bank” area that is a favorite forbidden skating mecca.

Such details are causing a buzz among skaters, many of whom flock from the suburbs to roll off Boston’s concrete treasures, teaching each other the differences between “ollies” (lifting the board off your back foot) and “nollies” (lifting the board from the front). Skaters talk about the individualism of a sport that has been growing in recent years, the discipline it takes to master a trick, the creativity they say they don’t find in “stick and ball” sports.

But their afternoons and evenings are an endless cat-and-mouse game of settling on a site and getting chased away by police, who insist they have to respond to neighbors’ complaints. Lema says that police have met, several times, with the Friends of Post Office Square, “and that's really the only issue they have.”

“It's not robberies, murder, and aggravated assaults,” he said. “It's skateboarding.”

As Conservancy leaders scanned the planned parklands beneath the Cambridge edge of the Big Dig, along the unused, industrial banks of the Charles, a skate park seemed a perfect solution, said Noah Stockman, the project manager. It’s hard, he said, to figure out what to put beneath a highway, where grass won't grow. But skaters like the site, because the ramps will protect them from rain and snow and beating sun. Engineers are intrigued, because some of the constraints, such as restrictions on how low they can dig around the massive concrete posts, make for creative thinking.

And some skaters are pleased that the park seems designed with their demands in mind, without the prefab elements that dot some smaller skate parks in Boston and the suburbs.

There’s only one potential problem, some say: Great as the park may be, some of Boston's topography is too irresistible to give up. And skaters worry that police will become even more aggressive about kicking them out of their favorite locations: the Financial District spot they know as “Window Ledges,” the Korean War memorial in Charlestown.

“I think it's kind of sick,” 14-year-old John Wisdom said of the park last Saturday, during a break from zooming on four low wheels across the dry bed of the Copley Square fountain. “But I don't want to be put in a cage.”

The answer is to make the park appealing by taking the skaters' ideas, said Zach Wormhoudt, the nationally known skate park designer commissioned to draw up plans.

While there are skating devotees in their 30s and 40s, the average skater is a boy aged 12 to 14. So the California-based Wormhoudt was intent on soliciting young people’s ideas for the design, listening to them at public meetings, and tramping with some of them through the site, which is now a Big Dig staging ground piled high with metal pipes and concrete barriers.

The teenagers sent passionate e-mails, suggesting intricate details, a few with the admonition, “put this in or die.” They sat around tables at the Boston Public Library, forming shapes with yellow modeling clay. They raised money: Cori Mintzer, 17, a rare girl skateboarder from Swampscott, begged her student council to let her hold a bake sale and said that serving on a design committee has made her certain she wants to be a landscape architect.

The results, many skaters say, is an architectural plan whose sickness, in skater’s jargon, is detailed and vast. The park will be divided roughly in two, to cater to two competing schools.

One side is “street,” which mimics typical urban features: concrete in the form of steps and park benches, a triangle to approximate the way a tree root can buckle a sidewalk, the ledges that suggest the Boston Medical Center.

The other is “transitional” or “vert,” a West Coast style of skating inspired by the gravity-defying potential of drainage pipes and empty swimming pools: dramatic, tall curves, including a bent tunnel, lit from the inside, meant to invoke an ocean wave.

It could draw thousands of skaters and BMX bike riders from around the country, sparking tournaments and national attention. Jody Stoddard, 34, a BMX rider and member of the Conservancy's design committee, said he’s taken road trips to skate parks in Toronto and upstate New York, and can see Boston becoming an East Coast skater haven.

“It's definitely going to put the city of Boston on the map of skateboarding,” said Jon Evjy, 21, who works at a Commonwealth Avenue skate shop called the Underground.

The park would be state property, but the Conservancy -- and probably the skaters themselves -- would have a role in maintaining it. Turnpike Authority Chairman Matthew J. Amorello supports it, as do numerous other local politicians.

And it will include space for spectators, who already flock around some of Boston's most popular skateboarding sites. At Copley Square on Saturday, a couple of dozen skaters in braces and baseball caps took turns doing tricks across the fountain, surrounded by wide-eyed toddlers, regular skateboarder-watchers, and a former chemistry teacher who admitted fascination with the sport, even though he had almost been run over, at times, by bikes and skateboards in MIT’s Infinite Corridor.

Not everyone is unfriendly to their sport, skaters say. But many of them hope that a real, attention-grabbing park will give them a touch of legitimacy that a city bred on hockey and baseball hasn't always bestowed.

“I hate to say it, but a lot of people are ignorant, and they have the wrong perceptions about skateboarders,” said Will Burstein, 15, of Winchester, who is on the skate park’s design committee. “They'll go up to you and sort of look at you and act like you shouldn’t be here. We’re just out to have fun and use benches and ledges and curbs, the way other people would. But in a different way.”