KB9VBR Antennas: Project E.L.F. – The history of communicating with submarines underwater – #HamRadioQA at https://www.youtube.com/watch?v=eC1cqwGkOwY
Now, with high dielectric constant and magnetic permeability antennas, these electromagnetic frequencies do not need large structures. Plus gravitational sensor array methods have progressed rapidly in the last couple of decades. So now you can use gravitational location methods, at least for submarines, inside the earth (caves and underground facilities), and shielded facilities. There is a world-wide effort to determine which parts of these low frequencies, of natural origin, are “electromagnetic” and which are “gravitational”. The answer seems to be “a bit of both”. Some of the “quantum entanglement” experiments are identical to the gravitational time of flight (the speed of light and gravity are identical) correlation imaging methods, used for 3D mapping sources like seismic waves, tsunami, planets, and flows in atmospheres, oceans and magmas. Joe Weber and Robert Forward both wanted to use gravitational signals for communication and engineering. Robert Forward particularly encouraged combining gravity and electromagnetism by simply finding a common set of units, methods and instruments. That has happened now.
Richard Collins, The Internet Foundation