Hidden Cave
I remember going to a museum that had a artificial cave. It was super basic, just a big loop. But my brother and I found a secret passage up to a second floor area that was much more natural looking with really rough terrain and it was almost completely dark.
I later began to think that the memory didn’t really make sense, so it was probably just a dream. Years later I went to grad school in a neighboring town and went to a local museum. I walked into a cave exhibit and realized it was the same one.
The secret passage was just a barely disguised stairwell and while the second story was more rough, it was fully lit. I think the second story was closed when I went as a kid, so they had the lights off, and had blocked the entrance.
It was kind of disappointing, because the memory was pretty magical. It was kind of like finding out Santa Claus was just a postal worker.
Story credit: Reddit / Foldedpencil