The creepiest thing that ever happened to me was when I used to work at the zoo when I was 14. We were opening up the zoo, and we have a building with some aquariums in a hallway that winds around inside the building. I was the first person in the building right in the morning (btw no windows) and it's really dark because none of the lights were on, the only light was coming from the aquariums. I saw what I can only describe as a white mist from around the corner of the aquarium. I went and chased after it and it kept moving farther away from me. It had no shape so I wasn't thinking ghost but it was just a blob really. At the end of the hallway, there is a doorway that leads to a tiny room that leads to the atrium where all the birds are flying around. The white mist went into the tiny room between the two exhibits and I followed it in there. The moment I opened the door, it wasn't there anymore and I thought that I just imagined the whole thing, but when I turned around to the leave the door was locked. I tried the door to the atrium and it was locked as well. I sat there for a sec, not panicked but more confused when the aquarium lights went off and it was pitch black. Then I started to scream, but only for a couple of seconds before the lights turned back on, and the doors unlocked. As I was leaving, I saw one of the veteran zoo keepers working in one of the aquariums and I told him what just happened and he said that he's seen the white mist before too. That was the last time I opened up at the zoo
