Having done cave rescue, I'll argue with that. Nothing transmits through rock or earth. That's why antennas are on roofs and satellites bounce signals.
If the government gives you a million, I'll help for half.
The digital transmissions had a much higher penetration measurement.
If you want to go into a cave? I could fix you up. It would take some work but yes, I could make you transmit from a cave.
What would I need? Well there are two ways. Could rig a series of repeaters to the entrance and then go out. Or I could drill a hole to the top (or close to the top). Like I said I don't know.
Yes, if you go into the bowels of the Earth you might lose coverage. I was speaking about the city. In an office building.