All the possibilities seem extremely unlikely, but I’ll add one more just for fun: We currently equate “advanced civilization” with “large scale”. But what if

All the possibilities seem extremely unlikely, but I’ll add one more just for fun: We currently equate “advanced civilization” with “large scale”. But what if there was a small-scale advanced civilization? Small enough to evade detection using the crude geological tools at our disposal. Imagine a species that was both intelligent (like humans) and immortal (like jellyfish). With unbounded lifespans, they could do work that would take millions of humans - just over a much longer period of time. With no urgent need to reproduce, their population could stay small. Perhaps with long life comes enough accumulated wisdom to exercise restraint. They might even marvel at how a mortal species could ever achieve civilization when knowledge has to be re-learned by every generation. Over millions of years of slow civilization they might have achieved great sophistication and advanced technology, concentrated in one small region that we either haven’t found yet, or which has been subsumed beneath ocean or tectonic plate. Perhaps a changing climate forced them to leave Earth, or to hibernate somewhere, peeking out every few million years to see if the conditions are right for their return.