Mention slacklining, and most people won’t have a clue what you’re talking about. Even those who see it wonder what it is they are doing.

“I tell them I’m just tight-rope walking and leave it at that,” says Matt Blank, 20, of Costa Mesa, Calif.

It isn’t a circus act but rather an art. It is a sport combining balance, concentration and walking a line, and it has become a world-wide phenomenon.

“It’s growing so fast, it’s just amazing,” says Maria Quinones, who, with husband Ric Phiegh, established the first online site for slacklining in 2002.

Kyle Breakey, a Western Washington University senior, shares those views, saying that slacklining also is popular in Bellingham but a lot of people still don’t know what it really is. Though they’ve likely seen someone doing it on any warm, sunny day at Boulevard Park or on the WWU campus.

“It incorporates strength with balance. It’s a big challenge. It’s really fun. It’s really social, too. You set up a line, you’re in the middle of campus and a lot of people who walk by and are curious stop by,” says Breakey, who started slacklining three years ago — just before he started learning how to rock climb.

Germany, Britain, Norway, Austria, Canada, Poland, Argentina and Italy are among the countries with slackline communities, aside from the U.S., where it originated among rock climbers.

The sport’s birthplace is Yosemite at Camp 4, where a permanent slackline is said to be in place. Climbers in the late 1970s started balancing and walking on the loose chains alongside parking lots.

Adam Grosowsky and Jeff Ellington, climbers who lived in Yosemite Valley, took it further and became the first to walk on nylon webbing, or so the story goes.

The sport evolved from there.

A slackline can be stretched between two trees three to five feet off the ground or between canyons thousands of feet up.

Scott Balcom, Phiegh’s brother, became the first to slackline the Lost Arrow Spire in Yosemite. He walked 55 feet across a 3,000-foot drop — with a safety line attached, of course. This is also called highlining.

Slacklining is different than tight-rope walking because the line moves and a balancing pole is not used.

“Since the line can move, it’s all about body position,” Blank says.

And it’s tough, Breakey says, which is part of the appeal.

“It takes a few sessions, a few hours of just working the line to get the lower leg muscle coordination so you can even stand up and take a few steps,” the 22-year-old student explains, adding that newbies start with spotters so they can hold onto another hand or shoulder until they can get a feel for the movement of the line.

From there, they can work on standing on the line without holding onto to anyone, then walking without help, then walking across the line, then walking backward or turning around upon reaching the end. Next, they can try lunging, jumping or walking sideways on a slackline, according to Breakey.

“You’re only limited by your imagination after that,” he says. And your ability to balance.

Breakey’s repertoire of tricks includes kneeling, starting from his knees, and walking sideways.

“It’s more like surfing when you’re walking sideways, only the line is moving a lot more,” he says.

Blank, who has been slacklining for two years, works at Rockreation, an indoor climbing gym in Costa Mesa where most of the employees have tried it.

Though it started with rock climbers, slacklining is said to be spreading among surfers, skiers, snowboarders, rowers, martial artists and gymnasts.

Quinones calls it “the ultimate crossover sport,” which helps to promote concentration and balance.

Meg Lord, 22, and Nathan Gerdes, 27, both of Costa Mesa, join Blank in demonstrating the sport at Estancia Park in Costa Mesa recently. Blank stretches one line 40 feet between two trees about 5½ feet high and another 30 feet and 3½ feet high.

Barefooted for better grip and feel, Blank runs and jumps up on the shorter line. On the higher line, he nimbly climbs up.

Then with the balance of a cat — at least most of the time — Blank walks the line, methodically putting one foot in front of the other while looking down at the line in front of him. One time he answers his cell phone while walking the highest line and doesn’t falter. Other times he loses his balance, tries overcorrecting and falls gracefully to the ground, unhurt.

Because of the concentration factor and its ability to relieve stress, slacklining also has been called “moving meditation.”

“For me, it takes a lot of concentration to walk a line,” Blank says. “When you’re doing that, you’re not thinking of anything else, so it does have a calming effect.”

How does Gerdes feel about slacklining? “I just think it’s fun,” he says.