Customers float in pitch-black saltwater filled tanks, sometimes for hours, to clear their minds, relax their muscles and, some say, to have an “out-of-body experience.” According to clients, the experience is unique, every session is different and the benefits can linger for days.

Floatation tanks have been around since the 1950s, but today people seeking stress-relief are discovering the powerful mental and physical effects of the tanks, and more centers are opening across the country.

“People will say it’s the most unique experience they’ve ever had,” said Eric Polcyn, owner of SpaceTime Tanks, 2526 N. Lincoln Ave. “All distractions are removed and there’s only you, [initially] researchers created isolation chambers to self-observe.”

Flotation tanks, also known as sensory deprivation tanks, are typically 8 feet long, filled with 10 inches of water and 800 pounds of dissolved Epsom salt. There is no light, sound or distraction inside the tanks.

“You stay relaxed for a long time after you get out of the tank; your whole system slows down,” said Keith Lutman,  computer programmer from Alexandria, Va., who has owned a flotation tank for 10 years. “That stays with you the whole evening, the next day or even longer, depending on how often you do it.”

Invented by physician and psychoanalyst John C. Lilly in 1954, flotation tanks were first used to test the effects of sensory deprivation, or the absence of visual and auditory stimuli, on patients. In the 1980s, researchers continued to use the tanks to study consciousness.

One reason some people use flotation tanks is to reach different levels of consciousness, Polcyn said.

The 1980 science fiction-horror film “Altered States” was based on Lilly’s sensory deprivation research. In the film, people in isolation tanks were under the influence of psychoactive drugs such as Lysergic acid diethylamide or, LSD, something Lilly investigated with his patients.

In 1983, scientists and doctors at St. Elizabeth Hospital in Appleton, Wisc., conducted a yearlong study of the effects of floatation tanks. They found significant improvement for a wide variety of problems, including anxiety, chronic pain, hypertension, gastrointestinal and cardiovascular ailments, migraines, tension headaches and heart surgery recovery.

Currently, there are flotation centers in most major cities in the country, and anyone can purchase a tank for personal use. Used tanks range from $3,000 to $10,000 at Floatation.com.

“It’s incredibly relaxing,” Lutman said. “One hour in the tank is like sleeping for four or five hours. You get into a relaxed meditative sort of state that is physical and mental.” Polcyn explained that every float is a unique and personal experience.

“You might have different things going on in your life or different within yourself,” Polcyn said. “We’re always changing and evolving, so each [time] gives you the opportunity to go through the mind-body experience differently.”

Lutman said he uses the tank once a week, but when he feels stressed he floats more often. He said it is hard to explain how it feels to someone who has never tried it.

In saltwater, the body is supported, so the tank creates a space where gravitational strain is diminished. In this space, the body is freed from environmental stress, and the mind is not distracted.

“When you get in the tank and close that door for the first time, it’s amazing the kind of mental chatter that comes up,” said William Faith, employee at SpaceTime Tanks. “I don’t think we really are aware of how much sensory input we get on a daily basis until we are away from it all.”

According to Polcyn, the closest comparison to floating is night diving in an ocean.

“It’s like trying to explain a flavor to someone who’s never experienced it,” Faith said. “You have to go through it on your own. It’s completely individual, as unique as the people who try it.”