I am writing a story in which two children, ages 12-14 years old, discover a portal to another dimension. The portal is roughly the size of an average door and has existed for five years prior to their discovery of it.
The location of the portal must fit all of the following:
- It must not be in a private home
- It must not be in a remote location, must be somewhere near a small city/suburb
- The children can trespass into a location, but there must be a reason why they will do it (best bet is to ensure that it's accessible from a public place)
- It must not involve passwords or secret codes; it must be immediately usable once found
- It must not be disguised as another object (ex. a poster)
So where could such a portal be? How could it be hidden for five years but suddenly found by two children?
I would prefer the portal being found in an indoor setting, but you can suggest outdoor places as well.