Hawkins National Laboratory

The Hawkins National Laboratory, operated by the U.S. Department of Energy, was a federal complex located in Hawkins, Indiana. It was most likely actually controlled by the CIA or NSA, and was one of several national laboratories which grew out of the scientific endeavors of World War II. It controlled several subsidiaries, including Hawkins Power and Light. The lab was headed by Dr. Martin Brenner until the events of November 1983.  Dr. Sam Owens then took over as Director of Operations.
In the mid 20th century, the lab conducted MKUltra experiments, and at some point began abducting children to experiment on. During one experiment, on November 6, 1983, a child test subject named Eleven made contact with the Demogorgon, accidentally opening the Gateto the Upside Down. For a year, an alien biological growth, stemming from the other dimension, consumed the lower levels of the lab. Despite the lab's attempts at containing the growth, the Gate was able to grow to a gargantuan size with a series of tunnels extending from it beneath Hawkins.
In 1984, an interdimensional entity called the Mind Flayer unleashed a pack of adolescent Demogorgons through the Gate, which resulted in the deaths of nearly all those present in the laboratory at the time. However, the Mind Flayer and his army were stopped when Eleven closed the Gate. Soon after, the lab was shut down following disclosure of the lab's involvement in Barbara Holland's death.