Look out your window, if you need proof: This county is flat.

There are some "rolling hills" in Howard, a cyclist would tell you, and Ellicott City has a bit of slope. But in the general scheme of things, to someone who's made it up Antisana, Otovalan, Cotopaxi and Toclaraju, this place is flat.

Then why does one of the nation's biggest schools for rock and mountain climbers--one that will help Girl Scouts dangle their way up the indoor training wall and lead die-hards up Everest--sit in an office park in Columbia?

The Pentagon sends people to Earth Treks to learn how to rescue people off rocks. Six-year-olds go there to turn 7. Its guides lead groups on expeditions up the world's most challenging peaks and ice slabs. In 1992, its first year, Earth Treks taught 250 people to climb. Last year, it taught 10,000.

Earth Treks' gym, opened in 1997 and the largest on the East Coast, is in the corner of a vast building on Columbia Gateway Drive. Two rust-colored walls reach 44 feet high. Upstairs are some smaller walls, for starting out. They are dotted with grips, wedges, knobs to step on or grab. Colored tape marks paths to the top, which are labeled for difficulty and named "Touch and Go," "Disgruntled," "Attack of the Killer Butterfly."

Ropes are cinched at the top and held tight on the ground by people called belayers, who urge their friends up the wall and give slack where it's needed.

The walls--in all, 13,250 square feet--are more than steep. Some actually slant fiercely the other way, so there's an overhang at the top. One can only be reached by climbing a fake boulder and bridging a five-foot gap. It is impossible to imagine anyone performing this feat, except perhaps someone with the arms of Mark McGwire, the height of Manute Bol and the poundage of a jockey.

Chris Warner says he can teach you how.

Warner, who founded Earth Treks, grew up in suburban New Jersey--a place about as flat as, oh, Howard County. When he was 15, he went on a five-day wilderness class, and immediately knew what he wanted to do, forever.

He'd drive with his buddies to southern New York, where they'd take the kind of fattish rope you use to keep suitcases on top of your car and take turns climbing to the top of a cliff. By 18, he'd climbed his first mountain, in the Grand Tetons, and by his mid-twenties he'd scale miles at a time.

Warner, now 35, is lean and tall, with black hair that's short all around, except for the tiny braid in the back. He spends half his time outside the country--sometimes with his wife, Joyce, who also climbs--and he still gets scared. ("When you lose your fear," he says, "something's wrong.") But he's at home outside. He spent the last six weeks climbing in Peru, and the first day back in Ellicott City, he broke his toe getting out of bed.

He moved to Baltimore in 1989 to direct an Outward Bound program, taking youths from the city, from prison, into the outdoors and outside of themselves. Now he does the same with bankers and housewives.

Jon Krakauer's best-seller "Into Thin Air" and the Imax film "Everest" tell the most frightening tales of the most elite and perilous climbs. But the climbing world reaches far beyond Everest. A recent survey by the Outdoor Recreation Coalition of America found that 400,000 Americans climb at least nine times a year, and that the number of climbing gyms grows each year.

According to the American Mountain Guides Association, which accredits climbing schools and guides, few places teach everything Earth Treks does: the indoor wall, outdoor classes and expeditions. Outfits in West Virginia teach climbing on Seneca Rock. Outfits elsewhere in the country--mostly Colorado--lead trips. But for Washingtonians who sit in their offices dreaming about snowy summits but haven't climbed anything more challenging than a flight of stairs, on the Eastern Seaboard only Earth Treks can take them through every step.

Tracy Swope, 37, of Columbia, saw an ad for Earth Treks in the back of Outside magazine and signed up for a climbing class at Seneca. Swope, a Marine Corps major, found she loved to climb, but it took some coaxing from Warner to get her on an expedition. Swope, you see, hates the cold.

She told herself, "I'm gonna do this once, get cold, get the T-shirt and that's it." But she found out that you don't notice the cold so much if you have good gear, or a good view--say, the sun shooting pink through a glacier across a broad, blue sky. Six treks later, "I'm pretty well hooked."

Swope gives a big chunk of the credit to the patience and skills of the Earth Treks guides.

"The whole thing about guiding," says Warner, who leads six to eight international trips a year, "is preventing as many unexpected things as possible." You've got to make sure the emotionally fragile climber is paired up with the tough one. That the bridge over the crevasse will support everyone. That the person who says he can make it to the summit knows what he's talking about. That you can cradle a climber from another group's expedition as he dies--as Warner has done--and still have the strength to go back and lead your own.

You've got to make sure you know just what to tell the person clenched with fear at the edge of a cliff with a thousand foot drop. Roll your shoulders, Warner will say. Deep breath. Make sure both your thumbs are pointing up. One thing at a time. "If I am super-positive," he says, "they feel like they can make the moves on their own."

With all his own triumphs, Warner sounds proudest when he boasts that Earth Treks has never had to give out more than a plastic bandage or ibuprofen on its trips. He makes an unlikely statement for an adventurer: "The last thing we are is a bunch of thrill-seekers."

CAPTION: Chris Thomas, left, Jon Irwin and Scot Heidtman work their way up one of Earth Treks' 44-foot walls, which are dotted with grips, wedges and knobs to teach climbing. The school is the largest facility of its type on the East Coast. Below, Thomas practices "bouldering" in the cave.

CAPTION: Signs on the walls help guide or inspire the climbers as they reach for the next handhold. At right, Jon Irwin practices on the strength board, which helps to strengthen muscles in the arms and hands for climbing.