John C. Lilly explaining his 1954 invention, the isolation tank, in a 1983 Omni interview:

“Omni:

Tell me the circumstances that led you to invent the first isolation tank.

John C. Lilly:

There was a problem in neurophysiology at the time: Is brain activity self-contained or not? One school of thought said the brain needed external stimulation or it would go to sleep–become unconscious–while the other school said, ‘No, there are automatic oscillators in the brain that keep it awake.’ So I decided to try a sensory-isolation experiment, building a tank to reduce external stimuli–auditory, visual, tactile, temperature–almost to nil. The tank is lightproof and soundproof. The water in the tank is kept at ninety-three to ninety-four degrees. So you can’t tell where the water ends and your body begins, and it’s neither hot nor cold. If the water were exactly body temperature, it couldn’t absorb your body’s heat loss, your body temperature would rise above one hundred six degrees, and you might die.

I discovered that the oscillator school of thought was right, that the brain does not go unconscious in the absence of sensory input. I’d sleep in the tank if I hadn’t had any sleep for a couple of nights, but more interesting things happen if you’re awake. You can have waking dreams, study your dreams, and, with the help of LSD-25 or a chemical agent I call vitamin K, you can experience alternate realities. You’re safe in the tank because you’re not walking around and falling down, or mutating your perception of external ‘reality.'”

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“The tank was unusual in that it was vertical and looked like an old boiler”: