For millennia it had been drifting across the vast emptiness of space.
"The Leech"
Phillips Barbee
It's stood for nobody knows how many millennia, and nobody's ever decided what it was, to begin with, except that somebody, once, filled a valley with concrete, level from mountain-top to mountain-top.
"Flight From Tomorrow"
Henry Beam Piper
Your race might destroy itself before your secret is rediscovered millennia from now, and the greatest wonder of creation be lost forever.
"The Short Life"
Francis Donovan