A sensory deprivation tank, or isolation tank, is a lightless, soundless tank filled with saltwater in which individuals can float. One such tank was present in the underground subsystem of Hawkins National Laboratory. A test subject with psychic abilities labeled Eleven was lowered into this tank for at least two secret government experiments. When Eleven entered the tank, her usual sensory experience was inhibited, allowing her inner psychic abilities to be greatly enhanced.
Eleven was ordered to spy on a Russian agent while in this state. This was a success, but in addition she unwittingly came across a monster from another dimension. Though Eleven panicked and the experiment was aborted, Dr. Martin Brenner made her return to the tank to make contact with the Monster. In the very moment contact was established, a tear in reality between the two dimensions was created, allowing the Monster to enter their dimension. Amidst the ensuing chaos, Eleven managed to exit the tank and escape the laboratory.
Becky Ives, the sister of Terry and aunt of Eleven, told Joyce Byers and Chief Hopper that as part of her involvement with MKUltra, Terry would be stripped naked and be placed in a sensory deprivation tank while on psychedelic drugs. It is unknown whether this would have occurred at Hawkins National or at another facility.
A few days after this conversation, Joyce, Hopper, Mike, Dustin, Lucas, Jonathan and Nancy made a makeshift sensory deprivation tank for Eleven to use, in attempt to communicate with Will and Barb in the Upside Down. This 'tank' consisted of a paddling pool filled with de-icing salt from the Hawkins Middle School reserve.