Spirit Orbs: Is the American Museum of Natural History Haunted?
Short answer: no, of course not. Still, I did manage to photograph a couple of "spirit orbs" while I was there:
This photo was shot in the Hall of Marine Life; it's an enlarged detail of this photo of the blue whale replica that hangs from the ceiling there. By increasing the contrast slightly and adding the power of suggestion in the form of a note with a couple of arrows, the reflection of the camera's flash off motes of dust turns into ghostly apparitions floating through the museum.
Spirit orbs are the most easily replicable alleged evidence of the paranormal. Why anyone would still invest belief in such things always mystifies me, but some people certainly do.