This one edges up closer to the limit of what can be obtained through open or declassified government sources: a list of VLF transmitters, their locations, and power. Before anybody starts sending me links such as this one: http://www.thornett.net/Rosliston_Archive_2009-11/VLF_Transmitters_List.pdf, or this one, http://en.wikipedia.org/wiki/Very_low_frequency#List_of_VLF_transmissions, read on a bit.
I think there is good reason to believe that much of the publicly available information is inaccurate or obscured. This is not a conspiracy, but an expected hangover from the Cold War as well as the obvious need to classify certain types of military capabilities. For example, Deming (2004) discusses the possible role of the TACAMO (Take Charge and Move Out) airborne VLF transmitters in producing the Hum. The TACAMO system protects the continuity and integrity of communications in the command structure of US forces, in the case of any attacks against its land-based VLF transmitters. There is no need for the public to know where these planes are at any moment, but their broad areas of service are known. I think it makes sense in this case to be skeptical of the commonly quoted numbers for transmitting power.
We don’t have access to internal documents, and so I can only speculate that the actual number of big VLF transmitters – in particular their broadcast powers – could be considerably greater than commonly assumed.
Feel free to send to me whatever you have on this topic, as long as the references and documents appear in the public record.