What are the Mysterious Brick Circles in San Francisco Intersections?

Spend some time walking around San Francisco, and you'll probably notice the large, brick circles decorating the pavement at some intersections. What are they for? Reported by Sarah Craig. Bay Curious is Olivia Allen-Price, Jessica Placzek, Paul Lancour, Suzie Racho and Julia McEvoy. Theme music by Pat Mesiti-Miller. Ask us a question at BayCurious.org. Follow Olivia Allen-Price on Twitter @oallenprice.