Flotation Therapy, sometimes refered to as isolation therapy or sensory deprivation therapy, is the popular practice of spending an hour or more, floating peacefully in a water and epsom salt solution. Epsom salt is used to make the water light, so that you're floating instead of sinking. 

This therapy has gotten popular in the last couple of years, but has been around since 1954, when Dr. John C. Lily, a neuropsychiatrist, began studying and acquiring research, which eventually evolved to the float tanks we know today!