Blue Granite Georgia Guidestones
It's believed the content is a guideline for future conservation of mankind, but the basis has not yet been deciphered. In addition to ten guidelines inscribed in eight different languages, the stones themselves are orientated and aligned to mark the 18.6 year lunar declination cycle, as well as the Sun solstices and equinoxes. The cap-stone also manages to keep track of the current day of the year with a sun ray passing through and provides a direct peering hole at Polaris. Also, a key-guide tablet can be found a short distance from the guide-stones speaks of a time capsule buried below.
Recent update: In summer of 2022, the Guidestones were demolished for safety concerns after an explosion destabilized the roadside monument. It's believed this might have been politically motivated controversy over the contents etched into the stones.