Rose Lozier is scared of heights, but she finds herself at Earth Treks Climbing and Fitness in Columbia, Md., several times a week, making her way up walls that are 42 feet high.

The server, massage therapist and student from Ellicott City says it is the adrenaline rush that keeps her coming back.

"It sounds crazy, but I just like the feeling it gives me," Lozier says.

In the last several years, more people like Lozier have become hooked on indoor climbing, a sport that uses simulations of rocks and boulders to give climbers a thrill similar to navigating mountains in the outdoors. The rise in popularity has led to a boom in new facilities throughout the country, and Earth Treks is taking advantage of the trend.

When Earth Treks opened its first facility in Columbia in 1997, there were around 40 climbing gyms in the United States, says CEO and founder Chris Warner. There were 414 climbing gyms at the end of last year, according to the Climbing Business Journal, a publication that follows the indoor climbing industry.

"The whole industry has just exploded," Warner says. "It started off for climbing enthusiasts, and then others would hear about it and find it cool as well."

Earth Treks quadrupled the size of its headquarters facility in Columbia in November. It has two other facilities under construction, including one in Baltimore's Hampden neighborhood. The company is also converting the old Sports Authority headquarters building in Colorado into what will be the country's largest climbing gym at 53,000 square feet.

The company sees even more new facilities in its future. Earth Treks in November merged with another climbing company, Planet Granite Climbing Yoga and Fitness, hoping that the new larger company will be able to leverage more resources to expand. The closings of big-box department stores and manufacturing facilities have opened up real estate opportunities. The Hampden facility is opening up in an old Sears distribution center.

The popularity of indoor climbing comes as the sport is viewed less as an extreme sport and more of a mainstream recreational activity, according to a report on the industry by research firm IBISWorld. Many are looking for activities that will improve their health but that they still enjoy. Climbing was approved as an official sport for the 2020 Olympics, which fans hope will open the sport up to even more people.

"Across the United States, increasing awareness of the need for exercise, weight control, wholesome nutrition and a healthy lifestyle among adults and children is having a positive effect on indoor climbing wall facilities," the IBISWorld report says.

People are also spending more on recreational activities like indoor climbing as unemployment has fallen and people have more disposable income, IBISWorld found.

Longtime climber James Rein says veterans like himself sometimes use the indoor facilities for training. Indoor walls can be better for building finger strength and practicing other techniques than natural rock faces, he says.

"Climbing gyms are also a great place to practice some of the safety techniques," says Rein, who is manager of market and consumer insights for the Outdoor Industry Association but not speaking on behalf of the organization. "It is a great place to work on skills."

The $4 million expanded Earth Treks facility in Columbia is next door to its original location. The old site had become too small for the more than 300 climbers that used the gym on any given day. The climbing walls, with their wood frames, were also outdated. The new walls are made of metal and wood.

The bouldering section of the new gym is much larger than at the old facility. Boulders are large, shorter rocks that don't require ropes and harnesses. The rocks are bigger and the climbing more explosive and powerful. Instead of climbing up in to the sky, climbers move more horizontally across the rocks.

There are also more walls with varying degrees of difficulty for climbers to take on at the new facility. The walls have "holds," where climbers place their hands and feet, that are color-coded by difficulty. The holds may be far apart, making a climb more difficult, or they might be on a wall that sits at a steep angle, also making it more challenging.

The new facility also offers yoga classes and has more gym equipment.

"When we first built the (Columbia) club house, there was no fitness and no yoga; it was really just a climber's warehouse," Warner says. "You used to have to have a Gold's Gym or other fitness club membership if you wanted to do other things. Now you have both. Now people can come and do what they want to do."

Earth Treks has built a climbing community in the two decades since it first opened. People from all walks of life come to the facility. They come in for an hour after work or during their lunch break. Sometimes they spend hours tackling the different walls. For some, indoor climbing has sparked an interest in taking the sport outdoors.

Kat Porter, an instructor at Earth Treks, started indoors and quickly caught the outdoor climbing bug as well. She recently returned from climbing trips in Spain and Colorado. She says she liked the physical challenge of pulling herself up the rock and the mental challenge of constantly strategizing the best route to the top.

"It is a full-body sport that requires mental participation at all times," she says.

In the bouldering room, 38-year-old Christine Case made her way across the low-hanging walls.

"It's a great challenge both mentally and physically," says the Ellicott City resident, who began climbing 12 years ago.

Children as young as 5 climb in a room for young people. Lozier brings her 6-year-old daughter, Isabella, to climb with her. Dressed in purple leggings and a pink top, Isabella made her way up a wall in the kids' room one day recently with her mother guiding from the ground.

Isabella moved up, quickly placing her hands and feet on pink holds along the way. But right before she got to the top, she hit a mental wall.

"You got this, baby girl," Lozier coaxed her daughter. "You've done this so many times before."

"No, I don't," Isabella said before making her way back down the wall a few minutes later.

Some days are just scarier than others, her mom says.