The latest "Stranger Things" Season 4 theory claims that the Russians are experimenting on mixing human DNA with Demogorgon DNA to make exploring The Upside Down safe despite its toxic environment.

The Season 3 end-credits scene saw the Russian soldiers seemingly "feeding" their human prisoners to their captive Demogorgon. However, according to the theory, the Russians are not using them as food but as test subjects.

They are using the humans as part of an experiment in understanding the Demogorgon and The Upside Down. The Russians want to find out how humans can explore the dark place without the fear of being attacked by the creature or dying from the toxic atmosphere.

According to Screenrant, the human prisoners, including the unknown American, "could be there to test the behaviour of the Demogorgon when in contact with humans." Likewise, the Russians could be experimenting on mixing human DNA with the Demogorgon's DNA to find a way to allow humans to survive in The Upside Down.

It is said that the American captive is being kept for a more important purpose once the Russians become successful in their experiments on the human prisoners. The American will be next, and he will serve as the Russians' "biggest weapon against the U.S."

However, the identity of the American remains a mystery. Fans have thrown in several names including Jim Hopper, whom they believe the Russians arrested after the blast at Starcourt Mall in Season 3. There is also the possibility that Dr. Martin Brenner is the American captive. This would explain why the Russians know about The Upside Down and the Demogorgon, which interestingly, is the creature that attacked him in Season 1.

Regardless, all of these are merely theories. Fans will just have to wait to find out more details about the Russians' purpose in trying to open The Upside Down when "Stranger Things" Season 4 arrives. In the meantime, they can stay on the lookout for possible spoilers from social media photos, given that the installment is already in production.

Filming for "Stranger Things" Season 4 reportedly started in Lithuania for the Russian prison scene. With any luck, leaked set photos can give away a few spoilers about the upcoming season.

Stranger Things
Noah Schnapp, Gaten Matarazzo, Millie Bobby Brown, Sadie Sink, Caleb McLaughlin, and Finn Wolfhard attend the premiere of Netflix's 'Stranger Things' Season 2 Getty