List of octagonal buildings and structures in the United States
Octagonal buildings and structures are characterized by an octagonal plan form, whether a perfect geometric octagon or a regular eight-sided polygon with approximately equal sides. Octagon-shaped buildings date from at least 300 B.C. when the Tower of the Winds in Athens, Greece, was constructed.
Source: Wikipedia — List of octagonal buildings and structures in the United States (CC BY-SA 4.0)