This sounds like a start to a horror movie, an A-plus-plus-plus internet hoax or one of the scariest things you can find in your home. A user on Imgur was horsing around in one of the rooms in his house when his little brother ran into a bookshelf. Turns out, the bookshelf could open up. Turns out, the bookshelf hid a secret spiral staircase that led to a secret crawlspace where a stranger was apparently staying. Yikes... if it's even close to real.
According to the Imgur user, here's what the bookshelf looks like when it's not opened up and here's a view of the spiral staircase:
Of course you have to go down those stairs, right! You can't possibly let it go. Secret rooms behind bookshelves are always cool. Well, the Imgur user did go down and after going down halfway he saw the crawlspace:
And here's where it gets absolutely creepy, again if it's true (which with the way the Internet works, you always have your doubts):
You would find this. Someone was living in our walls. IN OUR WALLS. That is my Halloween candy. That someone came into my room, took from my bag, and then brought back in here. Banana peel for scale. About 30 seconds after taking this picture, we realised what it meant and got the fuck out of there.
There could be many explanations for this: the Internet is a place where everyone cries wolf just for the lulz (what's up with the dolls, right?). Or somebody who found the secret room first could be pranking the person taking these pictures (what's up with the dolls, right?). Or there really could be someone staying inside the house (seriously, what's up with the dolls, right?). He says he's called the police. We may or may not hear from him again.
Either way, secret rooms behind bookshelves are almost always cool... unless someone you don't know is living inside it.