The Upside Down is an alternate dimension existing in parallel to the human world. The history of the Upside Down remains a mystery. Exactly how and why it came into existence, is unclear.
It contains the same locations and infrastructure as the human world. There is little proven knowledge concerning the Upside Down, as it remains shrouded in mystery. In spite of this, the dimension possesses a few observable properties and characteristics. Such as, light, gravity, and sound appear to function the same way as in the regular world.