Dunno about geysers, though it could be neat if energy will be just sitting in place like an oil in BC.
I was thinking of some sort of energy deposits, or unminable resource, which, though, will have some kind of machinery to extract and then process to energy, say 60EU/t.
Something like Thermo Plasma Generator or Hydrocarbon Power Plant
Those deposits also can be deadly if someone will go there, and highly volatile (like, say - if you make a hole in hollow, filled with energy substance, it will start to leak, and you either have to block that hole or quickly place machinery for extraction).
Also maybe OV scanner has functionality to detect those hollows along with resources.
Maybe it was already suggested, but topic title says "MOAR power sources")
