If you haven't yet streamed Netflix's latest buzzed-about original series Stranger Things, you need to carve out a few days to do so. That's because there hasn't been a sci-fi show so mysterious — and that will continue to keep fans guessing even after its finale — since the good old days of Lost.

Between its '80s nostalgia (including the comeback of Winona Ryder), a creepy monster, supernatural abilities and otherworldly components, Stranger Things has set the bar for science fiction series. It's pretty much all everyone can talk about, which means there are quite a few fan theories that have been floating around.

Just hear them out, because those who have completed it know there have been stranger things that have happened in the series.

Warning! Those who are still binge-watching or haven't started just yet might just want to bookmark this for later to save yourself from spoilers.

Here are some Stranger Things theories that will continue to pique fans' curiosity.

The Hopper Is Eleven's Father Theory

There is potentially more than meets the eye when it comes to the relationship between Hopper and Eleven. The series creators the Duffer Brothers even teased that season 2 of the series could include a possible connection between the two characters.

Now, fans are speculating that Eleven could really be Hopper's late daughter Sarah.

Despite the fact that Sarah passed away, as fans know, deaths can very well be staged. Actor David Harbour, who plays Hopper, even teased that Sarah may not have actually died of cancer in a Reddit AMA.

There is a scene where Sarah has a panic/asthma attack, but maybe she is just reacting to something that she saw in the Upside Down.

Even though Eleven called Dr. Brenner papa, there is a good chance she has been programmed to think he is her father. Little is known about Eleven's true origins. It is curious that Hopper did know to leave out Eggo waffles for her, as is his newfound guilt at the end of the season.

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Hopper Is Really A Secret Agent

If the Hopper is daddy theory doesn't pan out, there is always the lingering suspicion that he is really working as a government agent. He does hop in the car at the end of the series to pay some kind of dues, but maybe he was on the side of the bad guys all along.

While this might be a stretch, it's strange that Hopper doesn't take Mrs. Beyer's frantic behavior to find Will more seriously at first. Maybe he already knew his outcome.

He also isn't killed when breaking into the Hawkins National Laboratory, which might seem odd since he starts to become a real threat to the research. Why would Dr. Brenner keep him alive if he wasn't using him for some other purpose? He later gives up Eleven's location to Brenner, which doesn't make him so much of a hero after all.

The Upside Down Is Now Unlocked

Now that multiple people have gotten into — and in some cases, escaped — the Upside Down, there is another theory making its rounds on Reddit that says another portal will open to further unlock this dimension. The Upside Down itself, as a place, has not really been explored to its fullest, so there is a chance there will be other ways and places where the show's characters can come in.

This might include the boys going to look for Eleven, or Will somehow going back and forth between the real world and the underworld.

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Since Will coughed up the slug in the finale, maybe he brought the Upside Down to his small town.

There Are More Monsters

Pay attention, and you will see that Hopper saw eggshells later in the series, alluding to the fact that there is now more than once monster lurking in the Upside Down. This would make total sense since there needs to be more creepy crawlers now that Eleven destroyed the main one in season 1. Will's slug also teases the fact that there are more or other kinds of monsters in this parallel universe.

There is also a theory that Eleven created the monster herself. She says, at one point, "I am the monster," not to mention, the only way she could kill the demogorgon is by destroying herself.

via GIPHY

While anything can happen during season 2 of Stranger Things, we will continue to speculate as more details and fan theories start trending.

Stranger Things is currently streaming on Netflix.