What spooky television series has an alternate universe called "The Upside Down"?

The spooky television series with an alternate universe called the “Upside Down” is Stranger Things.

Created by Matt and Ross Duffer, Netflix’s Stranger Things premiered in 2016 and follows a group of children in Hawkins, Indiana. The Upside Down is a dark, dangerous parallel realm that mirrors Hawkins, but is covered in vines, spores, and otherworldly decay.

The realm is connected to Hawkins through gates created or opened by supernatural experiments involving Eleven and Hawkins National Laboratory. Creatures associated with it include the Demogorgon, Demodogs, and the Mind Flayer. The characters use ideas from Dungeons & Dragons to name and understand these threats.

A common mix-up is calling the Upside Down simply another planet or a literal version of Hell. Within the show, it is presented as an alternate dimension linked to Hawkins, making it central to the series’ science-fiction and horror mythology.