"As soon as any person enters SCP-860-1, the door automatically closes. From inside SCP-860-1, the door appears attached to an infinite concrete wall, and is locked. No attempts to break the door from outside, with personnel inside SCP-860-1, have been successful. Attempts to break the door from inside SCP-860-1 have resulted in [REDACTED].The trail inside SCP-860-1 usually leads to another door attached to another infinite concrete wall. This second door leads to the normal destination room of the door on which SCP-860 was used. "

Yeah, It's theme for a forest, in which you get in by using SCP-860, a key to a door. I hope ya'll like this! :)
And don't get killed by monster inside. Interesting thing: if you're a girl, this forest will be a lot kinder to you. It's not a joke, it's a fact.