Disposing of nuclear waste
I recently read that each year, nuclear power plants in the US produces 16,534 tons of high-level radioactive waste. This waste must be stored for 10,000 years before it becomes safe, longer than any civilization has ever lasted.
To be responsible and safe then, we must consider how to store this in such a manner as to protect future civilizations that may arise after the collapse of ours. These civilizations may have no history of ours, and may not understand things such as radioactivity. What then, can we do to prevent people from exploring these waste storage facilities and endangering themselves.
One possible way would be to make these storage sites appear as sacred areas, protected with strong magic. Through pictograms and universally understood signs of death, we could attempt to scare people away from what they would not understand. This is similar to what the builders of the pyramids in Ancient Egypt did to attempt to ward off thieves. If anything, all that did was serve to draw people to them. After all, anything so heavily protected must be valuable, right? Curiosity, even in the face of danger seems to be a fairly dominant characteristic of humanity.
Then perhaps the safest guard would be no guard at all. Simply bury it in an unmarked and uninteresting area, preferably arid or otherwise inhospitable.