A peer-reviewed study found that infrasound—inaudible low-frequency sound from pipes and boilers in old buildings—triggers measurable stress responses in people who can't consciously hear it, including higher cortisol levels and increased irritability. This may explain why some people report eerie or unsettling feelings in old buildings without realizing the cause. The researchers note this effect could feed into paranormal beliefs when people are already primed to expect haunting, but the infrasound itself is just physics, not evidence of anything supernatural.