the bedrock of the Earth itself... could be turned into a colossal radio station... an "immense subterranean grid".
Mad science, yes. With a touch of the end times, given the total nature of thermonucler war:
transmitting to distant submarines by way of the Earth itself, humans using an entire planet as an apocalyptic radio device.
An afterimage of Tesla, tuning the Earth for his purposes, resurrected for the Cold War's last generation.