So you have 4 million highly-radioactive used nuclear fuel bundles. They are safely stored 500 metres underground. It's going to take about 1 million years before they are safe to juggle.
How do you keep people from accidentally digging them up?
What kind of a sign do you design that says to someone 500,000 years in the future, "Stay away! This place is really, really dangerous!"?
There is a better than fair possibility that English will no longer exist as a language a few thousand years from now. The same is probably true for the rest of humanity's languages.
The Americans have actually thought quite a bit about this. This was one of the questions they considered when designing their Waste Isolation Pilot Plant — a deep geological repository — in Carlsbad, New Mexico.
They put a team of linguists, writers, anthropologists and other scientists together and set them the task of coming up with a warning sign that would be understood by humans far in the future. The group came up with some pretty incredible suggestions.
Cover the landscape above the repository with a landscape of 50-foot tall pointy spires at random angles. The idea was to make the area look like a giant foreboding thorn field. Nothing says "stay out" like massive spikes.
Another massive landscape, this time of blocks that are stacked really close together so you can't walk between them and can't be shaded from the sun by them.
Earthen berms built up in jagged angles to convey a forbidding place.
The first two options would be very expensive, the third more affordable.
As for signs:
Use Edvard Munch's the scream.
The classic stick figure getting fried by way lines.
Finally, onsite, the group suggested etching warnings in some of the world's major languages on giant pieces of granite. They also imagined leaving a blank space for any future culture to etch the same warning in its own language.