"Stranger Things" Season 5, Volume 2 finally reveals the origins of the Upside Down. Here’s how the wormhole works, what the Abyss is and how it all connects to Vecna.

The second volume of the eight-episode season drops Dec. 25 at 8 p.m. ET (5 p.m. PT). On Monday, the Duffer Brothers shared the final runtimes for the last half of the final season. Here's the breakdown of the episodes dropping on Christmas Day: The series finale arrives on New Year's Eve on Dec. 31.

The play reveals that the cave system was where Henry first ventured to Dimension X — now dubbed "The Abyss" in Stranger Things. That's the alternate dimension where the Mind Flayer and Demogorgons are from, as opposed to the Upside Down, which Volume 2 reveals is a bridge connecting the Abyss to our world.

On Christmas Day, Stranger Things fans finally discovered the series-long mystery of the Upside Down, an alternate dimension many believed existed in parallel to the human world

**Spoilers for Stranger Things Season 5 Episode 6 “Escape from Camazotz,” now streaming on Netflix**