It also raises the interesting question of if humanity is all but wiped out, how ethically responsible are we to warn the next race of Whoevers that roam the Earth? Should we even care about beings whose society has no knowledge of the universal symbol for radiation from our time? We treat other species differently than ourselves, what about future descendants of ours who are not-quite-human? What if they are alien visitors? You can really go down a rabbit hole if you throw out the assumptions about who or what creatures may come across the place. I think we should just assume these future humans will have technology at least as good as ours and hence will have instantly searchable archives of information to determine the meaning of our symbols. If not, there are bigger issues for them then some radiation in the ground.