Fans shared theories as to why Jim Hopper is alive in "Stranger Things" Season 4, and they involve The Upside Down.
The mystery as to how and why Hopper (David Harbour) could have survived the blast at Star Court mall in Season 3 intrigued fans. So much so, that they suggested several ways on how he could have escaped before Joyce blew up the machine
Most of the suggestions involve The Upside Down, which the Russians were digging a hole into in Season 3. Cinema Blend pointed out that the gate to the dark place was about to open prior to the explosion. Hopper could have jumped through the gate and ended up on the other side, which, unfortunately, landed him in Russia. This would explain his presence as the American captive in "Stranger Things" Season 4.
Hopper has a bit of knowledge about The Upside Down following his experience there in Season 1. He and Joyce entered the dark place through the gate at Hawkins Lab in order to save Will. Perhaps he knew that he would come out of the place alive since he already knows his way around it.
Then again, fans have their theory that Hopper escaped the explosion through the bunker. There was a ladder near the machine and he could have climbed down when Joyce was not looking. Hopper has the blueprints of the Russians' underground lab at Star Court Mall, thanks to Murray. He may have already calculated his chances of survival once he decided to give Joyce the go signal to blow up the machine.
The showrunners, The Duffer Brothers, already hinted that Hopper was alive when they reiterated the importance of his corpse. His remains were not found on the blast site, which teased that he is alive in "Stranger Things" Season 4.
Moreover, the Russian soldiers who got blown up at the start of Season 3 left a gooey residue or ash. Hopper did not leave anything after the explosion.
Regardless, whether Hopper escaped through the gate to The Upside Down or through the bunker remains a guess. Fans will just have to see for themselves how and why he is alive when "Stranger Things" Season 4 streams on Netflix.