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.
It wasn't protected by some kind of password or disguised, neither it was in a place one could hardly access, such as far as from the suburbs in which they live or inside a private home.
But my problem here is figuring out a good location to ensure such a scenario happens. Is there a way for a door-sized portal to another dimension stay hidden for five years but then be discovered by two normal children? The best answer in this case is the one regarding the best location/placement in order to maximize the chances of such a scenario happening, preferably why no one but the children would check out such a location.