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The Sensory Deprivation Tank
It is filled with salt water that is at body temperature, causing the user to "weightlessly float" in the tank. The tank is sound proof and light-less. Basically, all your senses are muted for you to be left only with your thoughts.
The user is supposed have reduced stress and be comparable to hypnosis.
To find out what would happen if the brain was cut off from all external stimuli, scientist John Lilly built the first sensory deprivation tank in 1954. Floating in warm water for hours in complete darkness and silence, Lilly began to experience vivid fantasies. "These are too personal to relate publicly," he reported later. The hallucinations of his test subjects were similarly difficult to categorize scientifically. This was one reason why his research did not take off.
Lilly later gave up scientific research and founded the firm Samadhi Tanks, which manufactured tanks for domestic use. On 1980 Lilly's work was the model for the film "Altered States". Having became something of a New Age guru, he died in 2001.