flotation tank 
noun
A lightproof, soundproof tank of salt water in which a person floats as a form of deep relaxation.But there are few other places imaginable that are more conducive to complete relaxation than a flotation tank....
- Though the harmonic devices remain pretty much unchanged, the softer, recessed electronic textures and a greater use of silence offer a deep listening experience that rivals the best flotation tank.
- A flotation tank is like a huge bath, about the width and length of a king-size bed.