Hidden San Francisco: What's beneath those brick circles at intersections?

SAN FRANCISCO -- As you travel on the streets of San Francisco, you'll find more than 100 circles of brick embedded in intersections around the city.

It turns out that they aren't simply decorative. In fact, the circles are indicative of something larger hidden beneath the street that could make all the difference if a massive fire were to break out.

Check out the video above to find out what's built beneath the roadway.

Hidden San Francisco is an exclusive ABC7 series uncovering secrets of San Francisco. Click here to watch more stories.