The issue of communicating something in a universal language to an unknown audience reminds me of the cover diagrams on NASA’s Voyager golden records. In that case, the challenge is the density of information that needs to be packed, as opposed to the simplicity and clarity of a radioactive sign (“here’s precisely how to read the contents of this record” vs. “you will die if you come close”)

https://commons.m.wikimedia.org/wiki/File:The_Sounds_of_Eart...