Jamie Harris was skeptical the first time the Treadwall was rolled into the climbing gear department where he works at Hudson Trail Outfitters. Harris eyed the large contraption suspiciously. As a climber and self-described "purist," Harris is an outdoorsman who wasn't about to take his pursuits inside. That was before he climbed the Treadwall.

Nestled between the climbing gear and the backpacks, the Treadwall stands about 12 feet tall, twice the height of Harris, and it looks like a treadmill. But where a treadmill has a flat, smooth surface for walking or running, the treadwall is bumpy, decked out with brightly colored knoblike protrusions of various shapes and sizes. And where the treadmill is horizontal, the treadwall is pure vertical.

"I couldn't believe this thing at first," says Harris, pointing to the knobs. "I thought it had nothing to do with real climbing." Each of the wall's wildly colored protrusions serves as a handhold. The knobs range in size from boulder (about the size of a baseball glove) to pebble (think half-dollar), and they are the only thing between you and gravity when the wall starts to move: When the wall does get moving, the only direction to go is up -- unless you feel like crash testing the three-inch-thick mats below.

Indoor climbing has hit the Beltway in a big way. Climbing walls are cropping up everywhere, from gyms and local sporting good and outdoor gear stores to large indoor centers devoted exclusively to the sport. When it comes to climbing walls there are a lot of variations to choose from.

The mid-afternoon lunch hour is a busy time at the ESPN Zone, a sports-themed dining and entertainment complex in Baltimore's Inner Harbor. It's a place where patrons come to eat hearty burgers, watch sporting events on a myriad of television screens and then hit the Arena, a section devoted to sophisticated interactive games. Over near the Alpine ski machine and the free throw basketball simulator sits one of the Zone's most popular attractions, a virtual mountain called The Rock.

"It's been a huge hit," says Nathan Moore, assistant general manager of the Arena section. "You should see the businessmen at lunchtime: The suit jackets come off, the ties are loosened and they're all over it."

The Rock is a climbing wall with high entertainment value, an automated behemoth with lots of bells and whistles. Scaling the two-person wall is an experience in social climbing. Each climber's name is entered into a computer where it is then displayed on a 25-inch television screen, along with a computer simulation of each climber's progress, for all to see. "It can get pretty competitive," Moore admits. "Especially when the crowds start cheering and rooting for people by name." With the busy weekend crowds, Moore has also seen the wall become an icebreaker, a place where climbers meet and hook up, often heading to the bar afterward for a drink.

"There is just something about the idea of climbing," Moore says. "It appeals to younger and older crowds. Men and women. I don't know what it is, but as soon as people see it, they want to try it."

Not to imply that The Rock is all fun and games. There is some serious fitness training going on here. Just ask Martin Coleman, its designer. "The Rock combines the adventure of climbing, the thrill of extreme sports with a natural balance of aerobic and strength training," says Coleman on the phone from his office in Bozeman, Mont. "It's a great cardio workout."

The Rock is completely programmable; you can set the speed, angle and distance. The wall can adjust from a 15-degree easy slab to a negative-90-degree "roof" incline, for maximum versatility and challenge. By changing the machine's angle and speed, users can create a workout that brings them to the brink of exhaustion, both physically and mentally. With these fitness benefits, it's not surprising that this virtual mountain has found a home at several military training facilities and at health clubs.

At Results, a health club in the District, The Rock gets its fair share of climbers. As a rule, gym climbers are not necessarily hard-core mountaineer types, they are more likely people who just want to get into shape. "We have trainers who can work the climb into your fitness regime," says Mika Buffington, facility director at Results. "It is very effective for cross-training. You can use it like a treadmill or bike to warm up, or as an all-over intense workout." Those climbing at Results often use the preset climbing programs, where, with the flick of a switch, you can set a course for the challenging inclines of Denali or El Capitan. "Keeping in mind, of course," says Buffington, "that El Capitan is a three-day climb, and this is only about a 15-minute workout."

Results is not the only area gym to add a climbing wall to its stable of high-tech fitness equipment. In a recent survey at the National Capital YMCA, members were polled about which new feature they would most like to see at the gym. The survey results said: Get a climbing wall. And they did. Forty feet long and 20 feet high, the new climbing wall was unveiled two weeks ago, taking over space in the racquetball court where the less popular golf simulator once resided. Early indicators predict the wall will be one of the center's most popular features.

The climbing wall at the YMCA is static. It's a sturdy, solid surface that offers plenty of room to climb both high and wide. It's the type of wall that requires a harness and ropes and a belayer, a person to spot the climber and hold the ropes. If this is more your speed, there are several places dedicated to this type of climbing.

Earth Treks in Columbia is the largest indoor climbing center in the area, with 15,000 square feet of climbing surfaces available. This broad expanse of indoor terrain features just about every type of climbing landscape imaginable, including a bouldering cave (short overhang climbing, no ropes needed), aretes (finlike corners to climb up), slabs and even a stalactite.

Also in this category is the everpopular Sportrock, a climbing gym with two area locations. Sportrock has gone so far as to coat its walls with a textured finish and then streak them with water to re-create the look and feel of being out on the rocks. The impressive 40-foot-tall building (Alexandria location) has steep inclines, arches, roofs and towers to climb. Sportrock and Earth Treks offer an alternative to traditional gyms, appealing to both serious climbers and those just looking for a good workout.

Back at Hudson Trail Outfitters, Jamie Harris has made peace with the Treadwall. Harris is in serious training these days, using the Treadwall to work on difficult holds and foot positioning in preparation for his next big climb in Patagonia, Chile. "I never thought I would be climbing indoors," says Harris, as he ascends the swift-moving landscape. "But what can I say? It really works."

CLIMBING THE WALLS

The following is a list of area locations for climbing walls and facilities. Climbing costs vary by location, from no charge at sporting good and outdoor gear stores, to a $14 day pass at Sportrock and Earth Treks. There is an additional cost for gear rental where appropriate. All gyms listed offer non-member day passes(ranging from $10 to$16), except the National Capital YMCA, which does not offer day passes for non-members.

Stores:

GALYANS -- 12501 Fair Lake Circle, Fairfax. 703/803-0300.

HUDSON TRAIL OUTFITTERS -- two locations: 9488 Arlington Blvd., Fairfax, 703/591-2950; and 12085 Rockville Pike, Rockville. 301/881-4955.

R.E.I. -- 3509 Carlin Springs Rd., Bailey's Crossroads. 703/ 379-9400.

Health Clubs

GOLD'S -- 2900 Clarendon Blvd., Arlington. 703/527-4653.

NATIONAL CAPITAL YMCA -- 1711 Rhode Island Ave. NW, 202/862-9673.

RESULTS, THE GYM -- 1612 U St. NW, 202/518-0001.

Climbing Centers

CITY OF ROCKVILLE CLIMBING GYM -- 805 Avery Rd. (Rockville Civic Park/Mansion, at the junction of Baltimore and Edmonston roads), Rockville. 301/217-052.

EARTH TREKS -- 7135-C Columbia Gateway Dr., Columbia. 800/254-6287.

SPORTROCK I -- 14708 Southlawn Ln., Rockville. 301/762-5111.

SPORTROCK II -- 5308 Eisenhower Ave., Alexandria. 703/212-7625.

Entertainment

ESPN ZONE -- 601 E. Pratt St., Baltimore (at the Inner Harbor). 410/685-3776.

CAPTION: Jesus Lugo of Madrid spends part of his U.S. vacation climbing The Rock at Results, a gym in Northwest.