The region's pagan traditions speak of many gods, but the most powerful of all was the god of thunder and lightning, Perun. There are also plants in the region named after , as they grow in areas where lightning struck.
It is said that the stone forest also has many plants unique to that location.
There are specific symbols used in people's homes to this day which represent this god, according to Wikipedia these symbols represent ball lightning:
![Image](http://upload.wikimedia.org/wikipedia/commons/thumb/f/f4/Thundermarks.svg/320px-Thundermarks.svg.png)
Gromoviti znaci or thunder marks such as these are ancient symbols of Perun, which are often engraved upon roof beams of village houses, particularly in Eastern Slavic populations, to protect them from lightning bolts. It is conjectured their circular shape symbolises ball lightning.
Just scratching the surface here, but perhaps the region has known some spectacular lightning and thunder events, and created these sort of giant fulgurites?
The current theory elaborated by local researchers suggests gas bubbles caused these shapes while the region was underwater. I have not checked it, I think it it might need translating from Bulgarian, but on the surface seems more of a hypothesis.