Massimo degli Eredi found a very interesting Wired article:
American Stonehenge: Monumental Instructions for the Post-Apocalypse, devoted to one the most enigmatic monuments in the US.
The Georgia Guidestones are huge slabs of granite, inscribed with directions for rebuilding civilization after the apocalypse. Only one man knows who created them—and he's not talking. The story surrounding the monument is definitely mysterious and the
full Wired article is well worth reading.
And for those who'd like to learn more, there is also
a complete (even if dated) downloadable guidebook.