I just finished reading this article from wired magazine about uvb76, a Russian numbers station. I'd heard of this before, but the part that really tweaked my mind was this part:
"... Next to the tower are a blue shed, a green metal hut stuffed with wires and electrical equipment, and <b>an ancient stone structure</b> that’s also overgrown with moss. And there appears to be a large underground facility: The muddy pitch on which the tower stands is riddled with metal cylinders (presumably ventilation shafts) rising out of the ground, and there is a very small pink building that looks like the entrance to a descending staircase. Also, there’s a door that’s partially ajar on the side of the stone structure. If you open it and peer inside, you’ll see a black hole where there must have been a ladder several years or decades ago, and if you drop a rock in this hole, it will take about a second to reach the bottom—whatever is down there is at least 32 feet belowground."
The emphasis is mine. What exactly is the 'ancient stone structure' located at the site of the original radio tower? This, to me, is much more interesting than what the radio signal means.