It's a tank with water in it.
Here's some info I found:
Floating, floatation, float therapy...whatever you choose to call it, it is quite simply the most incredible means of stress relief and relaxation available. We will go into further detail on this page, but essentially the tank contains 10 inches of super-saturated salt solution, creating an environment similar to the Dead Sea which enables the individual to float effortlessly on the surface. The water is heated to skin temperature (93.5 degrees) so that once settled, it is virtually impossible to distinguish between parts of the body that are in contact with the water, and those that aren't, in effect "fooling" the brain into believing that the person is floating in mid-air.
Unless you are an astronaut, this is the only situation you will encounter where your body is completely free from the harmful forces of gravity...every single muscle in the body is allowed to totally relax. With ear plugs in and, if you wish, the tank's interior lights off, the quietness and the darkness allow the mind to drift into the deepest state of relaxation possible. The buoyancy created by the dense Epsom-salt solution effectively removes the effects of gravity on the body, and brings the individual close to an experience of total weightlessness. A massive amount of brain-power is used every second in order to deal with the huge strain that gravity places on one's body. As the body is now totally supported, there is little for the brain to do and every muscle is allowed to totally relax.
With no commands needing to be sent out, the logical side of the brain is rendered redundant, and its activity slows down until it synchronises with the creative side. This leaves the individual in a dream-like state, akin to the thoughts experienced just before you go to sleep. In this state, the brain releases vast amounts of endorphins.
While the state of relaxation may be deep and profound, the individual's brain stays dreamily alert. To get technical, the brain gradually shifts from its usual "alpha" state to generate theta waves, the state-of-mind that Buddhist monks try to reach through hours of meditation and years of training. You can achieve this in a matter of minutes just by lying down! Best of all, the effects of floatation are cumulative - the more regularly you float, the better it gets.
The Float Session
Each tank is situated in a lockable private room and changing area, equipped with an invigorating power shower for use before and after the session. There is a light inside the tank for beginners, or for those who may not feel secure in the dark although we have found that almost all users (even those who previously claimed to be claustrophobic) are floating with the light off within three sessions. As with the internal light, the door to the tank is fully controllable from within, allowing the individual to choose whether to float with the door open or closed.
After a quick shower, the individual enters the tank and ambient music is played for the first ten minutes. 50 minutes of golden silence follows, and as the session draws to a close, the music is played again to make the user aware that they should leave the tank.
