Stairway to the Sky
One of the many things that San Francisco is a city of is staircases. Most of the time they are an assist up a hill or between levels that are too steep for a street but fine for a person on foot. "Bridges of elevation" one might say, but here and there throughout the city they can be found as entryways. Invitations to unseen places. Portals to the little reminants of the wildlands that are impossible to eradicate.