This Place is Not a Place of Honor

What is the timeless and universal symbol (not just independent culture but species-neutral) for "Caution: Extreme Danger"? Or rather, what are current bipeds conceptions of the answer?

This article is about "The challenges of marking buried radioactive waste for 10,000 years." It's just fascination the watching modern humans suddenly pulling themselves out from their worldy political, cultural, and even current scientific biases and imagining (seriously, dealing at the international diplomatic level) with how to communicate to future, maybe not even human, sentient beings. Worth a read, for a change of thought pace.
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