The Upside Down is an alternate dimension existing in parallel to the human world. It contains the same locations and infrastructure as the human world, but it is much darker, colder and obscured by an omnipresent fog. The Upside Down is devoid of human life, instead being overgrown with ropy, root-like tendrils and biological membranes covering practically every surface. At least one recognizable animal, a humanoid predator, was native to this dimension, while ash-like spores floated in the air.
In 1983, a gate to the Upside Down opened at Hawkins National Laboratory following experiments on a telekinetic child. As a result, supernatural forces began to bleed into the local town of Hawkins, Indiana.