I listened to a great podcast this morning (99% Invisible) about messages. There's a nuclear waste site in New Mexico called WIPP. When it was constructed, the government approached several specialists to ask them to design a message/monument/label/warning about the dangers of the site that could effectively communicate the danger of the site for a minimum of 10,000 years.
There are a lot of difficulties in such a proposal. Keep in mind that 10,000 years is twice the total length of recorded human history. Hell, it's longer than some people think the Earth has existed (lol). Language, symbols, etc. may not behave the same that far in the future.
The most hilarious proposal given was Ray Cats. The idea was to genetically engineer cats to change color in the presence of radiation, then release them into the wild so the phenomenon of cats changing color = danger would become known to people around the world. They'd compound that understanding by writing songs and stories/parables about the dangers of color changing raycats.
I'm curious what people on this site would suggest for such a monument.