Stranger Things Season 5: What is the Abyss?

The following contains spoilers for Stranger Things Season 5, Volume 2.

"Everything we have ever assumed about the Upside Down has been dead wrong," Dustin (Gaten Matarazzo) says in Stranger Things Season 5, Volume 2. And how right he is.

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The latest batch of Stranger Things episodes blows the show's world-building wide open, revealing what, exactly the Upside Down is.