Outside, the spa's hallways are cloaked in the usual Zen-like hush, but here in our treatment room, a storm is raging. We lay blindfolded, with 100 pounds of flaxseed sacks draped on top of us and our limbs pinned to a specially designed chair. Thunder cracks in the darkness, sending a jolt down our skeleton that rattles us from teeth to toes. Our body is still quaking when a fine mist begins to rain down, coating our skin with a slick of water. Then comes the sound of drums -- faint at first and finally strong enough to make our chest vibrate like we've swallowed a pair of subwoofers.

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• The World's Wildest Spa Treatments

Relaxed yet? Travelers who haven't been to the spa lately -- and given the state of the economy, we don't blame them -- might be surprised to discover what passes for pampering these days. Indeed, many spas -- including the Taiz Sensorium we're experiencing at Tucson, Ariz.'s Miraval Resort & Spa -- now tout offerings that are a universe away from spa classics like facials and massages. Daring spa-goers can try anything from having their body tapped with a wooden hammer to taking a (literal) chill-out session in a room where snowflakes fall from the ceiling. Even spas that are sticking with more-traditional techniques are increasingly pushing them to new and often mind-boggling extremes (Hawaii's Grand Wailea Resort recently rolled out a 10-therapist, $2,000 "20-Hands Duo Massage"). And the extreme-spa movement's reach isn't limited to the U.S.; Parisian outfit Dans le Noir le Spa offers rubdowns in pitch-black rooms -- a potentially tricky arrangement that becomes a snap when all the massage therapists are blind.

The spa world has been struggling to work out the kinks ever since the recession hit. While the industry enjoyed a rapid expansion during the boom years, about 7 percent of U.S. spas have closed their doors since 2008, and pros say that even the survivors are struggling. According to the International Spa Association, a trade group, nearly one in four spas reported a drop in revenue from September 2010 to March 2011. To keep the white-robed masses coming, experts say, some spas are hoping to set themselves apart with one-of-kind experiences -- a task that has become increasingly difficult as once-unusual options, like watsu (an in-water treatment) and lomilomi (a Hawaiian massage technique), have found their way into even the most pedestrian of spas. Wooing today's seen-it-all spa-goers requires creativity, says Stephenie Handley, assistant spa director for the Grand Wailea: "Our guests are a little more difficult to please these days."

Of course, developing a new offering -- and training therapists to perform it -- isn't cheap, and many of these "signature" treatments end up being some of the priciest (the 50-minute Sensorium runs a cool $250). "Froufrou" or trendy experiences also risk coming off as gimmicky, says Judy Singer, co-owner of Florida-based consultancy HFD Spa, who points out that, on average, about 70 percent of spas' revenue comes from the four basics: massages, facials, manicures and pedicures, and body treatments (wraps and masks). And while borrowing unusual treatments from spas across the globe has provided the industry with plenty of hits, things do sometimes get lost in translation, striking American spa-goers as unhygienic, uncomfortable -- or just plain weird.

Taiz Sensorium

Miraval Resort & Spa (Tucson, Ariz.)

$250 for 50 minutes

The Sensorium combines sound, vibration and aromatherapy -- and a didgeridoo -- to bring people to "nonordinary states of consciousness."

Photograph by Andy DeLisle for SmartMoney

Before we step into the treatment room, Kephart Taiz, the sprightly inventor and operator of the Sensorium, tries to prepare us for what's to come. "It's a little futuristic," he says with a grin. "Some people call it a contraption." We'll say. In the middle of the room is a reclining seat (think extra-cushy dentist's chair) surrounded by spindly adjustable arms, each of which is topped by a wooden sphere about the size of a billiard ball. Once we've stretched out on the chair, Taiz folds the arms around us so the orbs are pressed firmly into our feet, hands, shoulders and back, and -- just in case we still had some wiggle room -- "cocoons" our lower body in a pile of heavy flaxseed-filled pillows. We also get a pair of headphones, so we'll hear the blend of sound effects and music Taiz has chosen for us (including, it turns out, a cello version of Metallica's "Nothing Else Matters") -- even as the same sounds are translated into vibration and fed into our body through the wooden spheres.

So how exactly does such an unlikely device make its way into a luxury spa? According to Taiz, it began about eight years ago, when he started tinkering with sound-wave, or "vibroacoustic," therapy. Eventually, the idea evolved into the current Sensorium chair (bicycle parts are a key component of the device, says Taiz: "They're lightweight and available"), which Miraval's president and general manager Michael Tompkins first heard about when Taiz was still operating out of a friend's Tucson apartment. Tompkins says he was sold immediately and that the resort's guests also seem to be on board: The Sensorium has been consistently sold out since it was launched, says Tompkins. What's more, Miraval is the only place that has it. "We like to be cutting edge, and we like to be the first ones out there," he says.

Evan Cohen likes it too. The Hoboken, N.J., dentist was eager to climb into the Sensorium chair on a recent visit to Miraval. He says he's been to the resort at least 10 times in the past five years or so and was starting to feel like there weren't enough new experiences to experiment with. When he first saw the device, Cohen says, he asked himself, "What have I gotten myself into?" But before long, he was sinking into what he describes as "an almost dreamlike state." The sound and vibration of the Sensorium left him with the same sense of relaxation he gets from a meditation class, he says. "I didn't walk away with my life changed," says Cohen. "But I would do it again."

Of course, not everything about the treatment has been so relaxed. For the moment, Taiz is the only person at Miraval who can operate the device, so when he's not available, neither is the Sensorium (he says he plans to train other therapists soon). Coming up with a way to describe the treatment to guests boggled the resort's staff; Tompkins says its brochure write-up went through dozens of drafts. And creating an unprecedented experience also means coping with the occasional unexpected freak-out: Taiz says he's discovered that people who have lived through a major earthquake or have a tendency toward claustrophobia are sometimes upset by the sensations evoked by the machine.

Creativity can also backfire if the treatment or feature seems over-the-top or misleading to guests. The website for Qua Baths & Spa at Caesars Palace in Las Vegas promotes the "falling snow" of its chilly Arctic Ice Room as one of its signature experiences. But one customer, Melissa Imhoff, an X-ray technologist from North Syracuse, N.Y., says that description takes some liberties -- during her visit, the "snow" was actually foam. What's more, the little room was filled with a scent that sent her fleeing back to the more traditional areas of the spa. "It's not for me," says Imhoff. Qua, for its part, says the mint scent is intended to intensify the sensation of breathing in cool air, and that the room's snow machine makes tiny ice chips, but that when the chips build up in the machine, they sometimes turn to foam.

Spas hoping to add something unusual to the mix often look abroad for inspiration; after all, they can thank other countries for popular techniques like shiatsu and ayurvedic massage. But some practices embraced by other cultures don't work so well at home. The ubiquitous nudity -- often mixing men and women -- of international spas, for instance, doesn't fly with Americans, say experts.

American regulations can also get in the way, says consultant Singer, noting that domestic spas have to be more careful about making medicinal claims. A handful of enterprising companies tried to bring "fish pedicures," a treatment where small fish nibble away at spa-goers' dead skin, from Asia to the U.S. Everything went swimmingly -- until states starting banning the practice, citing everything from hygiene issues to animal cruelty (at least 10 states have instituted a ban).

But for some spa-goers, the issue is less about any failing of the more unusual treatments and more about the appeal of the pampering classics. Kathleen Haberthy tried the Sensorium on a recent trip to Miraval, and though the Indianapolis stay-at-home mom says she enjoyed the treatment, given the choice, she says, she'd opt for a plain old massage. "I just prefer the hands-on," she says.