Photo credit: alsuga.
Photo credit: artolog / Art Siegel.
The cathedral's striking design flows from the geometric principle of the hyperbolic paraboloid, in which the structure curves upward in graceful lines from the four comers meeting in a cross.
At 2:00 pm every day, a shadow of what appears to be a large female breast is cast on the Cathedral. This phenomena is affectionately known as the "two o'clock tittie" by students.
Others suggest that St. Mary's needs be viewed from the Southwest in the midmorning. Alamo Square is a good viewing location, and depending on the season, sometime between 9 and 10 in the morning is the appropriate time.
The outline of the breast is believed to resemble the breast of the chrome woman who reclines on so many truckers' mudflaps.
Urban legend has it that the Catholic Church sued the architect over the appearance of the breast, claiming that the appearance of a naked breast on the side of a cathedral somehow mocks the Church, which is reputed for being uptight about sexuality. However, extensive research shows no evidence that the church ever filed suit against Nervi or even threatened to.
Thanks to Brick for pointing me to this story.