Despite all the reports of ghost sightings (28 percent of Americans report having ghostly encounters), there’s zero evidence to support the presence of supernatural beings among us. Science may not prove the existence of ghosts, but it can help explain why people think they see ghosts in the first place.

In this video from Vox, paranormal investigator Joe Nickell identifies some of the phenomena believers may mistake for paranormal activity. One possible explanation is infrasound, or the sound waves that fall beneath levels of human perception. Though we can’t hear these noises firsthand, our bodies sense them in other ways. This can cause chills, feelings of unease and depression, and even hallucinations.

Other contributors may include sleep paralysis (when you wake up while your body is immobile and experience waking nightmares) and grief. There are also a few less common possibilities that aren’t covered in the video below: Mold poisoning, for instance, can lead to irrational fear and dementia. Suddenly, a visit from a poltergeist doesn’t sound so scary.

[h/t Vox]