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 futurecivilizations don't speak the same languages we do.