You walk into an old building, and something feels…off. You can’t explain it. You’re agitated, unsettled, maybe a little on edge, and there’s no obvious reason for any of it.

New research published in Frontiers in Behavioral Neuroscience suggests the reason might be simpler than anyone wants to admit. The culprit, according to researchers at MacEwan University and the University of Alberta, is infrasound—very low-frequency sound, below 20 hertz, that the human ear can’t actually detect. It comes from aging pipes, ventilation systems, traffic, and industrial machinery. It’s essentially everywhere, and most people have no idea they’re being exposed to it.

The study recruited 36 participants and seated them alone in a room with either calming or unsettling music playing. For half the group, hidden subwoofers pumped infrasound at 18 hertz into the room. Afterward, participants rated their mood, their emotional response to the music, and whether they thought infrasound had been present. They also gave saliva samples before and after.

Is It Ghosts, or Ancient Plumbing?

People exposed to infrasound came back more irritable, less engaged, and rated the music as sadder than those who weren’t. Their cortisol levels were higher, too. And none of them knew it was happening. Not one participant could accurately identify whether the infrasound had been running at all.

“Participants could not reliably identify whether infrasound was present, and their beliefs about whether it was on had no detectable effect on their cortisol or mood,” said senior study author Rodney Schmaltz, a psychology professor at MacEwan University.

That last part is what makes this interesting. The body responded whether or not the brain registered anything. Something was happening under the radar, literally.

Schmaltz draws a direct line to the haunted building phenomenon. “In an old building, there is a good chance that infrasound is present, particularly in basements where aging pipes and ventilation systems produce low-frequency vibrations,” he said. Walk in already primed with the idea that a place is haunted, feel inexplicably agitated, and the story writes itself.

The longer-term picture is less clear. A single brief exposure spiking cortisol is one thing. Chronic exposure is another. “Prolonged cortisol release is not a good thing,” said Trevor Hamilton, a corresponding author and psychology professor at MacEwan. “It can lead to a variety of physiological conditions and alter mental health.” The researchers note that the sample was small and more work is needed, particularly across a wider range of frequencies.

Next time a basement gives you the creeps, check the plumbing before you call a medium.