Los Alamos National Laboratory has been working on Thru the Earth Communications since 1996 with support from the Department of Energy initialy to communicate and locate trapped miners. They have developed a very mobile device that can send both video and data communications thru 300 feet of vertical rock or 550 feet horizontality thru rock and soil. This is all based on the micro chips found in cell phones.
An article is presented here
http://www.afcea.org/signal/articles...437&zoneid=222
You will notice it works in pairs with a surface device and a subterranean reciever.
Here is the patent
http://www.freepatentsonline.com/y2006/0148514.html
Lockheed Martin has also been working on devices as well
http://www.mwrf.com/Article/ArticleID/22911/22911.html They have successfully communicated voice to a depth of 1550 feet with data beyond this.
Nikola Tesla was also working on this in 1915 in conjuntion with his world communications station.
Lets say TMP developed this earlier for communications with their boltholes and underground bases. A small reciever is placed near the surface above the bolthole that "listens" for coded signals from Prime Base and this device then transmits to the reciever in the bolthole thru the earth. VLF is still in use, but just at a very limited range and Prime Base doesn't need to errect large masts to communicate with the boltholes.