> I don't know if subways would have fossilized if they were only around for 1000 years, 60-120 million years ago. I think you misunderstand. The extent and l

> I don't know if subways would have fossilized if they were only around for 1000 years, 60-120 million years ago. I think you misunderstand. The extent and location of the systems would matter more than the duration of their use. What we'd be looking for is the geological result of artificial tunnels filling up with sediment that then undergoes normal geological processes to solidify into rock.