Quote:
Originally Posted by dcarson
I remember reading that one of the most obvious long term things we would leave is old oil wells. Holes through millions of years of strata that are filled with concrete when played out. Leaves a odd looking trace.
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Large masses of relatively chemically stable metal might endure for some time. For ex, if somebody had a vault full of gold bars, it might take quite a long time before that looked 'natural'.
Large masses of silicate glass can last a long time, too, at least as far as chemical alteration goes.