It's the good old Semiotics 101 riddle of how to keep people away from a
nuclear waste dump for the next 10000 years, when clearly warning signs
wouldn't work since languages and symbols evolve over time, until the
signs surrounding the toxic dump is mistaken for an invition to go
treasure hunting in there.
Now Danny Hillis is trying to pull off a similar project to build an
eternal clock monument/stonehenge that will communicate the (Western)
concept of time to the future generations. That is until some nutcase
in the Soviet bloc decided to fire the entire USSR nuclear arsenal at US
all at once in the year 2025 and reduce the entire continent into a
nuclear waste dump for the next 10000 years.
How about that for a circular message? <g>
Article on Hillis' time machine:
http://www.wired.com/wired/6.05/hillis.html