the.com/infrasound
the sound too low to hear but loud enough to make you afraid
below hearingFrequencies under 20 Hz, felt more than heard
elephant callsHerds communicate across miles using infrasonic rumbles
haunting effect19 Hz can trigger dread and phantom sightings
natural sourcesVolcanoes, storms, and meteors broadcast it constantly
nuke detectionGlobal sensors listen for secret atomic tests