Skulls symbolizing human being, death, tomb, illness, history and danger could suit the task of creating a universal warning message for human being.

The danger with using skull is that it could be interpreted as a grave. Graves, especially monumental ones like pyramids in Egypt, though being protected and communicating 'keep out' messages attracted numerous robbers. These days they attract tourists. This should not happen with a radioactive waste site.

Having in mind the idea to use skull lets move on to our concept.