Rikki Tikki Traveller said:Put them a million years back and there wouldn't be much evidence of ANYTHING. No buildings etc are going to last a million years without upkeep. Relics and artifacts could have been destroyed in the final war so that there wasn't much left to find. Then add a million years of rust and plant life and there may not be anything to find anymore. Maybe there is SOMEWHERE, but we haven't found it yet.
Regarding bases on the moon or the other planets. Who's to say they aren't there but haven't been discovered yet by us? Several places on the moon could easily hide a base and we wouldn't have discovered it yet, especially if it was underground.
Would you even need a million years? When I came up with my campaign I thought 300,000 years would be enough to destroy signs of cities. I based it on something I read, but can no longer remember where, or even the sources reliability.
It's still a handwave I admit. I recon any ancient technological civilisation would have given itself away more in terms of using up the easy resources of Iron ore and fossil fuels that couldn't be replenished in anything like 300,000 years, preventing or making harder our own industrialisation.
But it's a handwave that I personally buy easier than the chance of so many races discovering Jump in such a short space of time, in such a small space.
You wait 300,000 years for the next bus to come along and 6 show up at once?
Just my own little foible.