On the western face of the Hoover Dam, the US Bureau of Reclamation installed a little‑known plaza in 1931. Branded Monument Plaza, its bronze winged figures and flagpole dominate photos, but beneath them lies a terrazzo floor etched as a celestial map. Designer Oskar J. W. Hansen intended it to record the dam’s completion within the 25,772‑year axial precession cycle.

The floor marks the ancient North Star positions: Thuban for the Egyptian pyramid builders and Vega, which will assume that role in roughly 12,000 years. Long Now’s 10,000‑Year Clock references the same precessional interval, making the dam’s plaque a 26,000‑year timepiece that bridges engineering and astronomy.

Reporter Alexander Rose revisited the site, secured archival photos and, after contacting historian Emme Woodward, obtained original blueprints. Those documents reveal the precise star‑position calculations Hansen embedded in the stone. With the plans now public, preservationists can interpret the monument’s intent, and future tours may finally explain its astronomical purpose.