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Disposing of deadly nuclear waste is hard enough, but all the effort is for naught if you can't stop people from digging it up again. This is especially tricky since radioactive waste stays dangerous for tens of thousands of years. Blogger Matthew Yglesias and Slate's Juliet Lapidos are pondering what kind of symbol could serve as a warning for millennia: "This seemingly simple conundrum (just use a radiation hazard symbol!) is complicated," explains Lapidos, "by the fact that such a trespass would prove lethal if it took place not only in 60 years but in 10,000 or 100,000." Yglesias argues that 'insofar as we manage to avoid a major civilization-collapse ... future people will probably be able to figure out that the danger is real." If we can't stop the collapse, he reasonably points out, what are we doing worrying about nuclear waste signs? But Lapidos notes that even if civilization stays intact, the passage of huge spans of time leaves prior humans a mystery to their descendants:

Although modern English grew out of Anglo-Saxon, Americans need either a translation or special expertise to make their way through the Anglo-Saxon epic, Beowulf. Radiation-hazard symbols, as well as other icons immediately recognizable to 21st-century observers, are similarly insufficient ... Even if future trespassers could understand what keep and out mean when placed side by side, there's no reason to assume they'd follow directions ... The tomb of the ancient Egyptian vizier Khentika (also known as Ikhekhi), for example, contains the inscription: "As for all men who shall enter this my tomb … impure … there will be judgment … an end shall be made for him. … I shall seize his neck like a bird. … I shall cast the fear of myself into him." It's possible that the vizier's contemporaries took Khentika at his word. But 20th-century archaeologists with wildly different religious beliefs had no reason to take the neck-cracking threat seriously...Hence the crux of the problem: Not only must intruders understand the message that nuclear waste is near and dangerous; they must also believe it.

Can we design a universal symbol for "keep out"? And how much energy should we put into finding it?

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