Every once in a while, there arises the arcane question of how to keep far-future people (robots? aliens?) away from radioactive waste dumps that will still be dangerous millennia hence. Juliet Lapidos at Slate discusses it here, and there are some glosses on it here and here. I've long been fascinated by this question, but in a minor irony something I once wrote about it (a review of Gregory Benford's Deep Time) is now lost, though perhaps retrievable in the Internet archives.
Based on my own experience with certain outdoor-adventure, rock-climbing people, I think it's going to be hard to prevent future explorers from getting into the fields of giant jagged spikes or other such features that are meant to send a universal message of Stay Away.