San Francisco has numerous mysterious brick circles implanted in seemingly random intersections all over the city. The first one can go without mention - but once you see a second, a third, and fourth... it's enough to make the most apathetic people curious.
It turns out that these mysterious brick circles are an important part of San Francisco, and the SFFD's, history. After the 1906 earthquake and fire destroyed the city the government created an emergency water system to prevent future disasters. The circles in the pavement mark the location of huge underground cisterns, each approximately 32' in diameter and containing several thousand gallons of water to be used as backup should normal hydrant lines fail.