The song "San Francisco", which was written for this film, is now a popular anthem in the city and is sung on earthquake memorial days.
To create the destructive nature of the earthquake, entire sets were built on hydraulic lifts and shakers which were then raised and rocked violently.
Clark Gable did not want to make this film but he was at the mercy of MGM studio head Louis B. Mayer, who had just paid off one of his numerous paramours.
Clark Gable and Jeanette MacDonald did not get along at all during filming, and avoided each other completely off the set.
Jeanette MacDonald personally chose Spencer Tracy for the second male lead. Previously he had mainly been cast as heavies; this role completely turned his career around.