That's the problem faced by Sandia National Laboratories in their report Expert Judgement on Markers to Deter Inadvertent Human Intrusion into the Waste Isolation Pilot Plant. link
Among their suggestions:
Three information rooms would archive detailed drawings of WIPP's chambers and the physics of its hazards on stone tablets. They would also provide a world map showing all other known waste repositories and a star chart to calculate the year the site was sealed.
One such room would stand in the center of the site. Another would be buried inside the berm, its only entrance a 2-foot hole to inhibit theft of the tablets, sealed with a 1,600-pound stone plug. The third room would be off site — perhaps inside the nearby Carlsbad Caverns.
Another solution discussed is coming up with toxic booby traps that, should an explorer get far enough, kills the interloper so that they don't inadvertantly destroy the planet.
And also lots and lots of pictographs. Anyway, I told you all that to tell you this: on the assumption that, 10 centuries from now, humans can still decipher English, they've also composed this rather poignant warning:
Nothing of value is here.
This place is a message...and part of a system of messages. Pay attention to it!
What is here is dangerous and repulsive to us. This message is a warning about danger.
The danger is in a particular location. It increases toward a center. The center of danger is here...of a particular size and shape, and below us.
The danger is still present, in your time, as it was in ours. The danger is to the body, and it can kill. The form of the danger is an emanation of energy.
The danger is unleashed only if you substantially disturb this place physically. This place is best shunned and left uninhabited.
Sending this message was important to us.
We considered ourselves to be a powerful culture.
