1780: "The Hoover Dam Angels"

Interesting Things with JC #1780: "The Hoover Dam Angels" – Visitors rub the bronze toes of Hoover Dam's 30-foot angels for good luck, unaware that the floor beneath them contains a star chart designed to reveal the dam's dedication date thousands of years in the future. The figures stand above an astronomical record that accounts for the slow wobble of Earth's axis.