Before you start reading the below post, please recall those "mystery hums", or weird engine sounds coming from beneath the ground. I have never heard any, but apparently there was a whole bunch of people who did.
Subterrene Vehicles Traveling Underground: Past and Present
- The Hum Noise
- The Rumbling Below: Strange Sounds of "Engine Noise" From Underground...
Subterrene Vehicles Traveling Underground: Past and Present