Because of the near eternal threat that certain toxic and radioactive waste poses, international researchers are looking into elaborate "keep out" signs for for dump sites to be read by distant generations of humans. One such option has been proposed by Patrick Charton of the French nuclear waste agency ANDRA. It's a disk made of sapphire that, when viewed under a microscope, would reveal warnings in text and pictures interacting them to leave the site alone, in case future civilizations don't speak the same languages we do.
Another advantage of this concept — the prototype for which cost approximately $31,000 — is that the disk and and data on it would last an estimated one million years. On of the other proposed (and presumably cheaper) markers comes from Cornelius Holtorf, an archeologist at the Linnaeus University in Sweden, and is literally a large 3-foot stone with the words "Do Not Dig" inscribed on it along with some details as to why.
These concepts are being developed to mark underground vaults, such as one being built in Sweden, where hazardous waste will be kept for long-term storage. Researchers involved in the project believe that finding a way to clearly convey the danger of the sites will be essential in preventing their opening thousands of years from now. They point to the fact that Egypt's pyramids were plundered despite being filled with dire warnings — ones that modern people simply couldn't read.
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