After some aimless browsing, I somehow came to be reading about the Jim Creek VLF facility in Washington State, used for communications with (or, more accurately, communications to) submarines. Even with miles and miles of wire antennas in the air, and even with more than a Megawatt out, the signal isn’t able to penetrate very far in saltwater.
This reminds me of the Skyking broadcasts carrying instructions to nuclear-equipped Air Force resources. There are both Emergency Action Messages and Skyking broadcasts, with the latter apparently having higher priority. They’re read live over the air and are, of course, encrypted messages.
They’re arguably similar to the various numbers stations, which broadcast information encoded with a one-time pad and are used to communicate information to people in the field. For example, see The Lincolnshire Poacher, which begins with the titular melody being played repeated, presumed to be an attention signal to aid in tuning. Many of the numbers stations use a recorded voice reading off numbers or letters. Some of what we know was confirmed when the US tried five Cuban spies.