Terrestrial Sonar

Reminds me of the "talking" drums used in West Africa[1]. Low frequency sound can travel distances measured in miles.

[1] https://en.wikipedia.org/wiki/Talking_drum

Much as I like a good story.

I call BS.

I tend to think to much energy would be needed to go the distance then reform in another drum.

[edit] So if all the energy is transferred it'll be the same loudness, but one would assume it's at least a square root of loss.

An ear on a train line is linearish with neither depth or sideways loss and still needs a lot of amplification i.e. ear directly on the line. (So it seems to me from movies.)