This is 99% invisible. I'm Roman Mars.

There's a well-known literary technique called check-offs gun, named after the Russian playwright.

Basically, if a gun appears in the first act, it better be fired in the following act.

There are no unnecessary details. You see a gun, that gun gets used to shoot someone.

I have a corollary principle of my own invention and it relates to movies where there is a scene

at a construction site, and it is this. If on the screen you see an ugly shaft of exposed rebar.

Somebody is getting impaled.

There's something about rebar that fascinates me. If nothing else because there are very few

things that invoke a fear of being skewered. My pathological preoccupation with metal reinforcement

bars dovetails nicely with a structure in San Francisco, I've kind of become obsessed with

a tiny bridge in Golden Gate Park. This is the Alfred Lake Bridge by Ernest Ransom.

This is William Lippman. He teaches architecture at the California College of the Arts,

and he was the first person to tip me off to the importance of this humble little structure

on the very eastern edge of the park. It's sort of the entrance to the park from the hate side,

from the kind of hippie-slacker hate area right at the edge of the park,

and sort of leading into the children's playground. It's not a place that most people really want to

linger. It is a spot for drug dealing and various illicit assignations. And that is Robert Corlan,

author of Concrete Planet, giving us a lay of the land. We're in the Albert Lake Bridge,

one of the earliest surviving reinforced concrete structures in the world.

The bridge was constructed in 1889. It may be the least appealing sort of monument of

architecture or civil engineering. On one side, it's cracked where the earth is sort of pressing through.

It's covered in mold and lichen. Inside, it's this kind of odd surrealistic tunnel of

...