The Upside Down is an informal name referencing a fictional alternate dimension. This location is a major setting featured in the Netflix original series Stranger Things. According to the show's creators, the Duffer Brothers, this otherworldy realm was originally identified as The Nether in script treatments. The characters in the series however, with no other prominent resources to rely upon, took to calling it the Upside Down. This name was partially inspired by the character of school teacher Scott Clarke, who tried to explain the concept of alternate dimensions to his students with a parable about a flea and an acrobat.
The Upside Down is a mysterious dimension that serves as something of a shadow of the regular Earth dimension. The environment of the Upside Down mirrors the Earth dimension in many ways, including topographical locales, and even specific locations such as residences or schools. However, there are not humans that are native to the Upside Down. The single greatest predator existing in this world is a massive creature dubbed a Mind Flayer. The Mind Flayer commands lesser creatures known as Demogorgons to obey its commands, and they serve as basically an army to a hive intelligence.
In the early 1980s, the staff at Hawkins National Laboratory were continuing to conduct experiments on human subjects, as part of a holdover from the government's MK Ultra days, which now also included the development of psychokinetic abilities. One of the labs' subject, Jane Ivers, also known as Eleven, was so powerful with her psionic abilities, that she actually opened a breach between the real world and The Nether. With this breach, the Mind Flayer became aware of the real world, and a demogorgon was sent to seek out prey. Through the course of events, a young boy was captured, and a teenage girl was killed - both of whom had been dragged into The Nether.
A year later, more creatures had manifested, and other members of the town of Hawkins became aware of the existence of the Upside Down. Jane Ivers, who had since escaped, and went into hiding, eventually returned to Hawkins as she knew that she was the only one who had the power to close the breach. With Sheriff Jim Hopper assisting her, they went to Hawkins National Laboratory, which had been overrun with "demogogs", bypassed the various threats and reached the gate chamber, where Ivers then expended a tremendous amount of psychic energy to close the gate separating the real world from the Upside Down. The Mind Flayer however, still maintained its perceptions of things taking place on Earth.