Located in Soma (South of Market) on the intersection of 5th and Mission sits the Old U.S. Mint. Constructed in 1874 and designed to keep out tunneling robbers, this stone building actually survived the 1906 earthquake and fire. One of the features of the building which helped to save it from ruin was the central enclosed courtyard that houses a well that was used to defend the structure. The windows melted from the heat of the surrounding fire, but the building was not engulfed in flames. Good thing because at the time it was housing a third of the U.S. gold reserves.
In 1937, it was replaced by a newer structure that is perched on top of a hill overlooking Market Street. The Old Mint was open to tourists until 1993, and just recently has been slated for a remodel to make it a museum by 2012.
– Source Wikipedia