Earth had been a great home to us. But, the Sun was revving up to devour the inner planets, so we figured it was about time we pack the whole place up and archive it. No danger to the human species: We’d scattered ourselves across the galactic plane. There was no getting rid of us now, short of obliterating the whole Milky Way.
But, we grew up on Earth. There was sentimental attachment. So, we scanned the place, layer by layer, teleporting atomic state into dense storage substrates that occupied invisible dimensions.
And, you know what? You find the damnedest things when you’re packing to move. Even after millennia of occupation by human civilizations, there were things we’d never seen. We were more occupied with getting off the rock, than getting into it.
Vampires? Real. Ancient underground cities? Not quite hollow-earth, but real. Unicorns? Eh, not so much. We found everything, no matter where it hid in deepest ocean or darkest forest. Like cat toys and change under the couch. We were astounded.