Gate

The Mothergate", also known as the "Rift" and simply "The Gate", was a gate to the Upside Down located in the underground section of Hawkins National Laboratory. The Mothergate was unintentionally opened by Eleven on November 6th, 1983; Eleven was directed by Dr. Brenner to make psychic contact with the Demogorgon, an action which tore apart the fabric of typical reality, forming a rift in the wall connecting to the Upside Down.
The Mind Flayer used the Mothergate as a means to spread the Upside Down's organic growth; the Gate's presence began affecting the nearby town of Hawkins, forming a elaborate series of tunnels beneath the town and spoiling crops. The Mothergate grew to a gargantuan size for a year, until November 5th, 1984, when Eleven used her abilities to seal it. However, this was only a temporary halt to the Upside Down's incursion, with further Gates being opened in 1985 and 1986.