Some animals, like elephants, use infrasound to communicate over long distances, while certain fish species actively avoid it. How humans respond to infrasound has been less clear. In this study, researchers played infrasound alongside music and found that although listeners couldn’t accurately detect the infrasound, their irritability and salivary cortisol levels rose — suggesting that our bodies may react to infrasound even when we can’t consciously hear it. That invisible reaction might even help explain why people report unusual experiences in places like supposedly haunted buildings.