I mean, testing showed it generally got the point across even if people didn't understand *why* it was dangerous

I'm curious what testing and what people. Unless it's an as-yet uncontacted tribe in the Amazon rainforest, I'm not convinced that they successfully made a universally understood sign of danger

https://en.wikipedia.org/wiki/Long-term_nuclear_waste_warning_messages People have put alot of thought into this exact topic and there's no easy answers

I'm well aware. Personally, I like to think of it from the opposite perspective; what message might we find that someone could have written 10,000 years ago that would convince us not to mess with something? The only proposals that work are ones that involve translating the dangers of radioactivity to new languages. Every physical marker is just *begging* for an archaeologist to discover why exactly they were constructed.

"Something something - anything if you're brave enough"

You think? A light emitter could be quite useful. If I am in a low-tech society, having a device or material that would emit light in the dark could be pretty desirable. It might confuse me though, because that "light emitter" doesn't actually emit any light at all. Maybe this ancient society was full of liers or maybe their devices are all expired and broken. Probably their warnings aren't worth anything either.

If your vial of Cobalt 60 doesn't have a flared base....well....I suppose it doesn't really matter. Have fun!

So like a Pioneer plaque but spelling out in pictogram form particle physics and nuclear decay