Fair warning: Major spoilers for Stranger Things season three ahead!
Stranger Things season three ended with the show's most gut-wrenching twist yet: Chief Jim Hopper, trapped in the room with a gate to the Upside Down that needed to be closed, couldn't get out in time and sacrificed himself to save the rest. Since then, fans have been clinging to any and all hope that Hopper isn't gone forever, and one theory suggests that he's still alive, but in danger: in the Upside Down.
It does seem a little unlikely that Hopper would be just gone for good, and as Reddit user Strangerthingsideas- points out, there are some major clues that point to him still being alive. The catch, of course, is that this theory isn't the popular "Hopper is 'the American'" one, but instead suggests that he escaped to the Upside Down.
"So when Hopper went [in to] shut off the portal they never showed his death, my suspicion is he didn't die and he hopped into the Upside Down Entrance because he has been in there before and knows how to survive," the theory suggests. Hopper does have more knowledge of the Upside Down than pretty much any other character aside from Joyce and Will, which would support this idea that he managed to get through before the gate closed. Although the Upside Down is nightmarish for sure, it still beats being obliterated by a sci-fi explosion of matter. Symbolically, it also draws another parallel or connection between Hopper and his adopted daughter El: at the end of season one, she too seemed to "die" as a result of an epic battle but was actually pulled into the Upside Down.
Plenty of details support the idea that Hopper wasn't actually killed by the explosion. In the season's first episode, the results of the Russian device are shown to be gruesome, with the remains of its disintegrated victims scattered on the floor. When we see Joyce staring at the spot where Hopper vanished, there's no remains at all. Eagle-eyed viewers might also notice that Hopper glances back at the gate right before the explosion, as if sizing up the possibilities.
Lending further credence to the theory: Hopper knows that it's possible to survive in the Upside Down — after all, 12-year-old Will managed it for a full week in season one, and that was without any foreknowledge of how the dimension works at all. With these extra years of knowledge and savvy, Hopper could have decided to gamble on waiting it out in the Upside Down until someone (Joyce and/or Eleven, most likely) can figure it out and come rescue him. This adds some weight to the loss of El's powers, too — if Hopper is trapped, that lends a bigger narrative weight and a ticking clock to her struggle to get her powers back and at full strength.
One final Easter egg that seems to point to Hopper's survival: in the middle of one episode, Murray rattles off his phone number — and it's actually a real, working number that you can call and get his "voicemail." On his rambling voicemail away message, there's a bit addressed to Joyce, telling her he's found something and has an update he needs to tell her in person. The most likely thing would be evidence that Hopper isn't dead, and if anyone could figure that out, it would be Murray. Until season four comes out, you can catch us over here hoping that Hopper will be back to rejoin his loved ones again!