I've heard of those things that tesla was trying to do by sending power into the earth using a 1 conductor transmission technique. Can the earth be used to send modulated signals of voice to be recieved somewhere else on earth like under the sea in a submarine?
Don't know about Tesla experiment, but the purpose you want is achieved by using Long Waves. Expecialy VLF, Very Low Frequency, with a wave length of 10-100 Km can literally follow the Earth curvature an penetrating the sea too at around 100 m depth. I'm not aware on any communication technique penetrating the Earth. An interesting thing to notice is that reducing the frequency also reduce the available bandwidth to modulate: some CW strategy are needed to send information other than plain modulation.