Theory

This is interesting, very peculiar. I'll give a quick background of Over the Garden Wall before I begin my reasoning to what the Unknown is.

AND SPOILERS FOR ASPECTS OF THE SERIES!

Over the Garden Wall is a cartoon mini series about two brothers who find themselves lost in a forest, trying to find their way home. The older brother is Wirt who is dressed in some sort of gnome costume. The younger brother, Greg, has a teapot on his head to dress up as an elephant. It is later revealed that they were dressed for Halloween.

In a flashback episode, we find out that Wirt and Greg climbed a cemetery wall to escape the pursuit of the police. (I can't remember if they were actually running from the police as I haven't seen this episode in awhile. The why they are running isn't important, but what happens next is.)

Wirt and Greg climb the wall and fall on the other side where there is a set of train tracks. Suddenly, there is a train coming and the two brothers jump sideways, tumble down a hill, and fall into a pond. They swim to the top and find themselves in The Unknown. The Unknown is a forest where creepy, fairytale-like things lurk.

And the brothers seem to be lost inside of it.

It's as if everything in the forest has devolved from the rest of society. The people that Wirt and Greg run into mostly don't have electricity, but instead carry candles and lanterns to light their homes. Back where they're from, Wirt and Greg have electronic light bulbs and running cars. This suggests that the forest is detached from society, or maybe from the real world entirely.

Now, I don't think that the brothers died and that this is their purgatory. I don't think this is all a dream, or that they're in a coma either. My personal belief is this forest is another world accessed by the reflection in the pond water.

Let me explain.

Everything was fine when the brothers went over the wall. It's not the wall that made them enter a whole different world, but the pond. Once they emerged from the water, everything seemed different. The trees, the animals, the moon. Yes, the moon. Although it may still be a crescent, this is a different crescent shape. A shape that is not possible in the real world unless it's a reflection of our moon.

So, the Unknown is a different world detached from our own, and the entryway is the pond. If that's the case, how come no one else has found it?

Well, people have found it. They were just never able to leave.

Remember Quincy Endicott from Chapter 5: Mad Love? In Chapter 9: Into the Unknown, which is the flashback chapter, one of the gravestones reads Quincy Endicott in the cemetery. Quincy found the Unknown and was never able to return, making the people think that he died.

This also explains why he is a little (a lot) insane. Who knows how long he was there for. Being alone for that long in those cursed woods would drive anyone insane. At least everyone else has human contact with each other, even the Woodsman has constant conversations with the Beast.

That being said, I think Quincy is the only other person who found the Unknown. But, this also makes sense. No one ever goes over the garden wall, which is actually a cemetery wall, anyway. There are train tracks there, and no one would travel to the forest that lies outside the town. Heck, they have woods in the town! So, why leave?

So, we have established how to get into the Unknown, and why no one has found it before. What else is there? How about: why don't Wirt and Greg question their own sanity after everything they see in the Unknown? I myself would think I'm crazy after I saw what happens in Chapter 2.

But, remember the first episode. The Woodsman explains that this place is cursed by the Beast. Plus, they had encountered a monster already, so they understand that this place has paranormal stuff inside. Also, these are children. Sure, Wirt is a young teenager, but imagination still runs wild. It's easier for them to accept weird things than adults. Which is why Quincy turned insane when he couldn't escape the Unknown.

The Unknown is another world accessed by the pond in episode 9. There is no end to the cursed forest because that's all the world is. Just the Unknown, which the Beast feeds off of.

But, more on the Beast another time. I have more theories on the Beast, the lantern, the turtles, and so much more! (The Ringing of the Bell phantom, Adelaide, and the dancing skeletons just to name a few more.)

Those are different theories for a different time. Peace!