A man will climb a mountain because it's there. He will climb a building, however, when it's not there. The mountain, that is.

Buildering, or the art of climbing buildings is a little-known sport that has been given greater public notice by climbs like Joseph Healy's ascent of Chicago's Sears Tower on Monday and George Willig's conquering of the World Trade Center last May. But those climbs, if you want to get technical about it, aren't really buildering at all.

"Buildering is more of a spur-of-the-moment thing," explained Willig yesterday from his home in Queens. "Very often you walk down the street, see a building that looks interesting and decide to give it a try." Just like that, with no special equipment, no advance planning, just what the good Lord give you.

"Climbing with mechanical devices doesn't give you the same sort of challenges most buildering does," Willig said."With the World Trade Center, the challenging part was the designing of the devices, figuring out what was to be done, not the climb."

Buildering apparently has been around as long as there have been to climb and climbers stuck in cities and unable to get at the real thing. Though estimates run into the thousands, builderers have no organization or lodge to join and no one really knows how many of them there are lurking in the shadows.

The name comes from bouldering, a form of practice-climbing done on small rocks that can be jumped or fallen off without serious bodily harm. One of the first printed references to buildering is in a 1930s volume called "The Night Climbers of Cambridge," detailing how devil-may-care undergraduate curfew-breakers took to climbing buildings to get back into the college after the administration had made the fences too horrid to attempt.

"It still has kind of fan outlaw appeal, the idea of doing something that's illegal or seems to be," said Doug Robinson, a California climber and builderer who was once slammed against a wire fence by a policeman who thought his scrambling distinctly suspect. "Most often it happens late at night, when a group of climbers come out of a bar and starts looking at buildings and challenging each other. It's pretty casual."

Given their ruthers, builderers prefer older, stone structures to builder on, complaining, as does Willig, that "most modern steel or glass building, very smooth faces without hand-holds, don't lend themselves to it."

What builderers look for are either jamcracks, tiny openings between three-quarters of an inch and six inches wide, where they can jam in a knuckle or a toe, or chimneys, larger spaces where parts of the body can be wedged in, all with an eye on upward mobility.

Since buildings tend to be the same all the way up, builderers tend to find them boring after a certain point and rarely attempt to go to the top. "It's the intrigue of the technical problem, like chess," explained Robinson. "I'll find a building just on the edge of possibility, but after I've figured out the first two or three moves there's no point in climbing all the way up. The fun is in playing the head games, getting the solution."

And even though buildering seems to be getting more popular, no one is afraid the sport will become supersaturated like tennis, with, said Willig, "people waiting around for new buildings to go up so they can climb them." Moreover, almost to a man, most present builderers say they prefer nature's own to man-made obstacles.

"Given the choice I'd go to Yosemite in a minute," said Doug Robinson, who nevertheless admitted, a bit wistful, "There's this route to the roof of the San Francisco Opera House I've been eyeing . . ."