For a more serious take on building things designed to last millennia into the future I like reading about the Long Now Foundation’s various projects such as the clock they designed to last 10,000 years.
One of the guiding principles behind the clock design was that they limited themselves to using materials humans have already worked with for at least 10,000 years so they could predict with confidence how such materials will hold up over a similar period of time. For example we know how a bronze object is likely to hold up over that period of time because we have artifacts from the Bronze Age, but we can only make educated guesses about what effect that period of time would have on steel and concrete.
