Then a funny thing happens. You pass through it -- the wall. You float through to the other side, and just as suddenly you're OK again. And now you don't mind the utter lack of sensation, the zero gravity, the darkness. You experience a feeling of submission, a surrender to the neutralizing forces of the Big Nada. You begin to love the tank.

The isolation tank -- aka sensory deprivation tank, aka flotation tank -- was created back in the 1950s as a government research tool to study the physical origins of consciousness. Scientists hoping to solve the ancient mind-body dichotomy saw the tank as a vehicle for liberating the brain from its routine workload, thereby creating access to a range of higher brain functions.

During the '60s and '70s, the sensory deprivation experience became popular with the counterculture crowd, due in part to the writings of Dr. John Lilly, the neuroscientist, author and acid-dropper who, while working at the National Institute of Mental Health, went looking for altered states of consciousness in the tank. The movie "Altered States" is based on Lilly's explorations.

Isolation tanks fell out of favor during Reagan's '80s, but lately they're gaining new respectability and popularity, as tank-equipped spas and flotation centers pop up around the country. Back in the psychedelic '60s, sensory deprivation was pitched to mind explorers as a porthole to higher consciousness -- whereas now it's peddled more as a quick fix for '90s-style stress.

Tank promoters say that flotation releases muscular tension, reduces stress, bolsters circulation, jacks up your immune system, heightens your powers of concentration and imagination and makes you generally happier and ennui-free. And, the pitch goes, these effects are cumulative -- that is, every time you float you increasingly fortify your body and mind.

So is sensory deprivation the ultimate cure for millennial malaise? Or a throwaway relic of misguided hippiedom? I went to the void to find out.

The H&H Flotation Spa is located in Tenleytown, a trendy shop-and-café zone a few miles north of downtown Washington. I arrive on a Sunday afternoon with a straightforward mission: Do a one-hour session in the isolation tank, soak up all immediate impressions, and jot them down for end-point journalistic rendering.

Wanda, my friendly H&H attendant, escorts me through the spa's maze of corridors to a dimly lit room in the back, where I get my first glimpse of the tank. Lilly's original creation was basically a big upright fish tank outfitted for human use, but today's tanks, as advertised on flotation Web sites, are sleek horizontal affairs made of heavy molded plastic or fiberglass. Some high-tech models, built more for recreation than isolation, come equipped with underwater stereos, video systems, aromatherapy vaporizers -- even intercoms.

My tank, however, is the stripped-down model designed for total sensory deprivation -- a plain, all-white monolith with a side hatch for climbing in and out, no high-tech accessories, no frilly accouterments. The tank is about as wide as a twin bed, and stands chest-high. Basically, it looks like a scale model of the space shuttle, sans wings.

The tank room is white-walled and completely bare, except for a shower stall set off in one corner, a bench and a towel rack. Wanda tells me to get naked and rinse off in the shower, then she'll come back and put me in the tank.

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