Quick answer
Union Square in San Francisco takes its name from pro-Union rallies held here during the American Civil War, not from a labour union. The Dewey Monument at its centre was dedicated by Theodore Roosevelt in 1903. The plaza sits on the roof of a garage finished in September 1942 — the world's first multilevel underground parking structure.
Full transcript
Whose Union
The square is not named for a labor union. It is named for the Union — the northern side in the American Civil War. In 1860 a minister named Thomas Starr King arrived from Boston to lead the First Unitarian Church here, and California's loyalty was genuinely uncertain: the state was young, far from Washington, and full of Southern sympathisers. King gave speeches on this block urging support for Lincoln, and thousands came. The pro-Union rallies held here between 1861 and 1865 gave the square its name, and it has kept it ever since.
The Column
The tall column in the center, with the bronze figure of Victory on top, is the Dewey Monument. It commemorates Admiral George Dewey's victory at Manila Bay in 1898 and the assassinated President McKinley, and Theodore Roosevelt dedicated it in person on 14 May 1903. The figure of Victory was modelled on a San Francisco woman named Alma de Bretteville, who was working as an artist's model at the time. Not long after, she married Adolph Spreckels, one of the richest men in the city, and later gave San Francisco the Legion of Honor museum.
What Is Underneath You
At the end of the 1930s, work began on something no city had built before: a multilevel parking garage entirely underground, designed by Timothy Pflueger, with a public park rebuilt on the roof. It was dedicated in September 1942, in the middle of a war, and that is the strange part. Materials were only released to finish it because it could double as a bomb shelter and an emergency hospital. It was the first underground parking structure of its kind in the world, and the design was copied internationally. So when you stand on this square, you are standing on a four level garage, on a lawn that is technically a roof. Look for the ventilation grilles at the edges of the plaza. Walk the square and look for the tell yourself.