Isolation tank

An isolation tank, sensory deprivation tank, float tank, float pod, float cabin, flotation tank, or sensory attenuation tank is a pitch-black, light-proof, soundproof environment heated to the same temperature as the skin. Flotation tanks are widely advertised as a form of alternative medicine. The tank is filled with 10 inches of water which contains enough dissolved Epsom salt to create a specific gravity of approximately 1.25–1.26. This environment allows an individual to float effortlessly on the surface of the water. The primary function of the isolation tank is to eliminate as many of the external senses as possible. They were first used in 1954 to test the effects of sensory deprivation.
History.
The isolation tank was developed in 1954 by John C. Lilly, a medical practitioner and neuropsychiatrist. During his training in psychoanalysis at the US National Institute of Mental Health (NIMH), Lilly experimented with sensory deprivation. 
Widespread commercial interest and use of the isolation tank didn't occur until 1972, when Glenn Perry, a computer systems programmer, began selling the first commercial tanks after he attended a five-day workshop by Lilly. 
In 1981, there were about $4 million in sales and rentals in the industry, and expectations were that the market would grow, spurred by the film "Altered States" (a film starring William Hurt as a psychopathologist experimenting with a flotation tank), which came out in 1980. According to one source in the industry, in the 1980s the rise of AIDS and the fear of shared water reduced demand for flotation centers. By 2013, flotation was more popular in Europe than the US, but had undergone some growth in the area around San Francisco; at that time a low-end tank cost about $10,000, and an hour-long flotation session cost about $70.
Floatation-REST.
Scientific research with float tank therapy generally uses the term "flotation-REST" (reduced environmental stimulation therapy) to refer to the technique. This term is preferred over "sensory deprivation" due to the fact that 1) the float tank experience actually enhanced sensory input from the body (e.g., cardiorespiratory sensations) and can also be conceptualized as a form of sensory enhancement and 2) the term "sensory deprivation" carries negative connotations of torture and hallucinations that have likely impeded legitimate research in this field. A study of 50 individuals with anxiety disorders only reported simple pseudo-hallucinations such as colored lights or abstract shapes (e.g., "“Visual lights and color", "very pretty and relaxing"”), which were described positively by participants and not mistaken for real stimuli.
Potential treatment for anxiety disorders.
Recent work by Justin Feinstein and colleagues at Laureate Institute for Brain Research has shown a very large effect size (Cohen's d = 2.15) for anxiety reduction in a cohort of 50 participants with anxiety disorders after a 60-minute float. Participants floated in an open fiberglass pool without an enclosure (which is optimal for comfort in individuals with anxiety) in a lightproof and soundproof room and had the option to float with lights on or off. The experience was well tolerated by the participants with no serious adverse events. Another study by this group found that a 90-minute float session greatly enhances attention toward, intensity of, and subjective pleasantness of cardiorespiratory sensations in 31 participants with anxiety sensitivity (i.e., individuals who are normally disturbed by feeling their heart beating, intense breathing, or other sensations that might be associated with a panic attack) relative to a control condition in which participants watched a nature documentary. The authors have hypothesized that floatation-REST allows individuals to forget associations between visceral sensations and anxiety and learn new associations between visceral sensations and relaxation. 
Neuroimaging.
Using functional magnetic resonance imaging (fMRI), researchers at Laureate Institute for Brain Research studied 24 healthy volunteers before and after three weekly 90-minute float sessions. The results demonstrated that flotation-REST reduced statistical dependencies or functional connectivity during the resting state within and between brain regions involved in mind-wandering (the default mode network or DMN). Decreased activity of the DMN has also been linked to meditation and psychedelics by other studies. The study also found decreased functional connectivity between the DMN and the somatomotor network (which here included premotor cortex, primary and secondary somatosensory cortices, the posterior insula, the supramarginal gyrus, and the paracentral lobule). Because the DMN is involved in self-reflective processes, this finding could be interpreted to mean that floatation-REST takes "the body off the mind." Although a similar pattern of reduced functional connectivity was also found for an active control condition ("chair-REST") in which a separate group of 24 healthy volunteers lied in a reclined chair in a dark, quiet room for the three sessions, the reductions in connectivity were not as pronounced for chair-REST as for floatation-REST.