Co-created by J.J. Abrams (Star Wars: The Force Awakens), Jeffrey Lieber (NCIS: New Orleans), and Damon Lindelof (The Leftovers), Lost ran on ABC for six seasons and remains one of the greatest network dramas ever made. Following a group of plane crash survivors who wind up on a mysterious, uncharted island, Lost hooked viewers early on with its many compelling mysteries that pushed the show into all sorts of different genres, from the supernatural to science fiction.

Unfortunately, while Lost’s large ensemble cast and use of flashback sequences made it a compelling drama all the way through, the series also gained a reputation in its later years for its numerous loose plot threads and raising too many questions without giving satisfying answers. To their credit, showrunners Lindelof and Carlton Cuse made attempts at providing closure to some of Lost’s biggest mysteries in the final seasons, but there were still quite a few that were never properly addressed by the time the series finale aired in 2010.

Although fans theories and supplemental materials have cleared up a good number of the following 15 Lost mysteries in the years since the show ended, the series itself never bothered to address them and so left them officially unresolved.

12. Why Must The “Rules” Be Followed?

In the shocking Season 4 episode “The Shape of Things to Come,” mercenaries working for Charles Widmore come to the island and kill Ben’s daughter Alex when he refuses to cooperate and leave the island with them. Ben later makes a comment about the rules being changed and at the end of the episode, we flash forward in time to a point where Ben is off the island and paying Charles a late night visit. He tells his nemesis that Alex being killed “broke the rules” and that he will be punished accordingly.

Ben and Charles aren’t the only characters to be bound to these allusive rules either, as Jacob and the Man in Black are prevented from directly interfering with one another because of them. In fact, the driving force of the final seasons has to do with the Man in Black seeking to break the rules and find a way off the island. But what are the rules, anyway? It’s never explained where they came from, what they actually are, or why anyone even needs to abide by them. From what we can tell, the rules were just a writing crutch for Lost to fall back on whenever the show needed to keep its bigger conflicts going and though they were teased as being a big mystery, they really didn’t amount to much more in the end than a series of gentlemen’s agreements.

11. Who Was The Other Christian Shephard?

In the early seasons, Jack Shephard’s deceased father Christian would appear to Jack on the island periodically, suggesting that he was a ghost or spirit of some sort. This theory was complicated greatly when Christian started being seen by other characters but eventually, we found out that he was really the Smoke Monster/Man in Black, who took Christian’s form between the first episode and when he assumed Locke’s form later on. While this explained away many of Christian’s appearnces, it didn’t account for all of them. Seeing as how Smokey couldn’t leave the island, the moments where Christian appears to Michael on the Kahana and when he appears to Jack in Los Angeles could not have been the villain in disguise.

Sure, both of these appearances could have been hallucinations on the part of Michael and Jack, but then there’s also the moment where Christian appears to Sun and Frank, despite being in Locke’s form somewhere else, which would mean he was in two places at once. The Smoke Monster has many supernatural abilities but this is the only time where he would have displayed this particular power. What we’re left with then is a few sitiuations where Christian=Smoke Monster doesn’t fit. We can probably just assume that the Lost writers came up with this answer much later on in the show’s run and didn’t account for it not being able to explain all of Christian Shephard’s appearances.

10. And What Happened To Christian’s Body?

The revelation that the reanimated Christian Shephard was really the Smoke Monster in disguise confirmed that the island didn’t resurrect Jack’s father. As we know, Christian’s body was on Flight 815 but since the Man in Black doesn’t need someone’s body in order to take their form (Locke confirms this), it’s unclear what happens to Christian’s body, as it’s never located. Did the Smoke Monster just throw it in some bushes somewhere in the jungle? Compared to other unsolved Lost mysteries, the whereabouts of Christian’s body isn’t really that significant but it’s still a plot hole all the same.

9. Where Did Mother Come From?

The Season 6 episode “Across the Sea” was meant to fill in details about Jacob and the Man in Black’s backstory and while it did shed light on their antagonistic relationship, it also left viewers with even more questions, which isn’t something a show like Lost should have been doing with only a few episodes left. The episode’s biggest question mark was the newly introduced character Mother, a woman who resided on the island long before Jacob and the Man in Black did. We learn that Mother kills the boys’ actual mother after she washes up on the beach and gives birth to her sons.

Although Mother gets points for being played by the wonderful Allison Janney, as a character she’s an enigma who seems to exist just to befuddle frustrated fans even more. We never learn anything about her backstory or why she would kill a woman and raise her children as her own. She’s the one who tells Jacob he’s special and that he must protect the island, but how did she get this job in the first place? Oh well, maybe the answers to these questions were never all that important to begin with (or at least that’s what we tell ourselves).

8. Seriously, A Cork?

The Heart of the Island made its first and only appearance in series finale and boy did it leave much to be desired. Located in a mysterious cave that can only be found by a current island protector, the Heart appears as a pool bathed in warm light with some sort of magic stone in the middle that stops the water from draining and holds the whole island together or …something.

We get that trying to visually convey something as abstract as the epicenter of a large island with mystical properties is by no means straightforward, but nothing having to do with the Heart or its cork makes much sense and the show never bothers to try explain what it is. Like much of what happened in Season 6, it’s just something Lost fans had to accept and move on with.

7. How Did The Others Get So Wealthy?

We’re sure there’s a perfectly reasonable explanation for this, but did you ever wonder how a group of people living on an island in the middle of nowhere since the 1970s somehow have access to seemingly unlimited resources and the ability to travel the globe? Remember, the Others wiped out the Dharma Initiative, which means they would have lost access to the group’s off-island resources.

Sure, the Others did once count the wealthy Charles Widmore among their ranks but he was exiled sometime in the 1990s, which means the group would have lost access to his financial backing. There’s clearly some other significant source of income driving the Others’ various schemes but whatever it is, viewers were never made privy to it!

6. How Did Walt Get His Powers?

Nowadays, he’s probably best remembered for that “WAAAAAAALT!” meme, but Michael’s son Walt also weighs heavy on the minds of Lost fans because we never got a satisfying explanation for his mysterious psychic powers. We know that his powers manifested before he came to the island, as he seemingly makes a bird crash into a window while at home in the episode “Special.”

While on the island, he does some pretty amazing things, such as warning Locke not to open the Hatch, suggesting that he already knows what will happen once he does. There’s no question that Walt is special but there’s never been a canonical explanation for why. During a panel at Comic-Con 2011, Damon Lindelof was asked why Walt was so special and the showrunner replied “because he is.” Sure, why not?

5. Who Were The Outrigger Shooters?

This is a unique mystery, in that Lindelof and Cuse have publicly stated that they had an answer to who the shooters in the other boat were during the outrigger sequence in Season 5’s “The Little Prince” but that it was cut from the script. The scene in question sees Sawyer’s band of survivors being shot at by a group in another boat, though the fight is interrupted by a time-jump before we can find out the identity of the shooters.

While not a big deal in the grand scheme of Lost mysteries, the outrigger sequence is emblematic of the show’s later seasons often shooting themselves in the foot in their attempts to balance introducing new questions while also trying to provide answers. There are many theories as to who the shooters might be and even though Lindelof and Cuse revealed that they had an answer in an earlier script, they’ve never actually revealed what it was.

4. Why Did Danielle Rousseau Forget Ben Linus?

Initially introduced as an antagonist for the plane crash survivors to content with (she captured Sayid and tried to steal Claire’s baby, after all), Danielle Rousseau became a much more sympathetic character later on when it was revealed that she had a daughter who was stolen by the Others as an infant. Specifically, Danielle’s daughter, Alex, was stolen by Benjamin Linus, who raised her as his own.

Now, one would think that Danielle would have Ben’s face burned into her memory – after all, she even witnesses him taking her child at gunpoint! But when Ben is first introduced and posing as Henry Gale, she doesn’t seem to recognize him. While you could argue that it was too dark out the night Alex was stolen for Danielle to see Ben’s face well, it’s hard to imagine someone as driven as her wouldn’t be well aware that Ben is the one who took her daughter, making this a pretty significant plot hole.

3. What Caused Pregnant Women To Die On The Island?

One of the most fearsome properties of the island that is established quite early on is that it is a deathtrap for pregnant women, with women consistently passing around the 100th day of their pregnancies. The Others recruit Juliet Barnes to the island to figure out why this happens but we never do learn what the reason is.

After the time travel plot, it’s revealed that women could safely conceive and deliver before the Incident in 1977, which discharged powerful electromagnetic energy. We can safely assume that the Incident was the direct cause of the subsequent pregnancy issues but since the details of why this happened are never discussed, the show’s explanation can be summarized as “just because.” Yeah, not really an answer there guys.

2. The Numbers

Arguably the most iconic Lost mystery outside of the “what’s in the hatch?”, the numerical sequence 4.8.15.16.23.42 is an important part of the show’s mythology and make numerous (no pun intended) appearances throughout Lost as both Easter eggs and important plot devices. While there is an official explanation offered in the “Lost Experience” alternate reality game — the numbers are part of the Valenzetti Equation, which predicts the time of human extinction — this never appeared in the actual series.

The closest we ever got to a proper explanation was Jacob’s list, with the numbers corresponding to specific characters, but as Jacob tells Kate, the list didn’t have any special meaning. It was just a list. Besides, this doesn’t help explain why the numbers were cursed for Hurley. It’s possible that the writers intended to do more with the numbers later on, but weren’t prepared for how much of a hold they would have on the show’s fanbase and just couldn’t come up with an explanation that would be satisfying enough. Whatever the case, the numbers remain one of Lost’s most frustrating, insufficiently explained mysteries.

1. Libby

Libby remains a serious point of frustration for Lost fans and encapsulates the show’s worst tendencies when it came to introducing mysteries and plot threads, only to drop them suddenly and never address it. Introduced in Season 2 as one of the few remaining survivors of the plane’s tail section, Libby reveals herself to be a psychologist and strikes up a cute romance with Hurley, only to be shockingly gunned down by Michael later in the season.

If this had been all there was to Libby, things wouldn’t have been so bad but the show frustratingly reveals that she was once a patient at the same psychiatric institution as Hurley and that they were there at the same time! That’s a pretty big, earth-shattering plot twist but Libby was killed off before we could find out anymore about her past and though there were plans to explore her backstory posthumously during later seasons, nothing ever materialized and Libby remained an enigma for the rest of Lost’s run.