ON JUNE 16, 1976, at 10:34 a.m., an excavation machine helping to widen the 7500 block of Venice Boulevard at Cardiff Avenue in Culver City accidentally struck an eight-inch underground gas pipeline. The resulting explosion sent fireballs into the sky and leveled the entire north side of the block. Eyewitnesses described a horrific scene, reporting that workers in a nearby drapery store “ran screaming into the street […] with their clothes and hair on fire.” The late edition of the Los Angeles Times ran the accident as its lead story, emblazoned with the headline, “WEST L.A. HOLOCAUST.”

The explosion, which killed nine and injured over twice as many, was the result of road workers’ estimations being “18 inches wrong,” according to initial conclusions by the National Transportation Safety Board. Test borings done in 1975 indicated that the Standard Oil pipeline was 18 to 20 inches below the road bed but in reality, it was far closer to the surface when the trenching machine plowed into it. Earlier in the ’70s, the American Public Works Association had already been discussing a “uniform color code” system to help label the locations and type of different wiring and piping. The Venice Boulevard blast accelerated that process and within a few years, a standardized coding system emerged, still used to this day.

Odds are, you’ve seen this system at work but have paid it little attention, especially since it often takes the form of colorful but cryptic regulatory graffiti: lines, shapes, and figures, drawn on the ground with chalk or paint. Most laypeople wouldn’t be able to decipher their meanings but Roman Mars and Kurt Kohlstedt want to change that. In their new book, The 99% Invisible City: A Field Guide to the Hidden World of Everyday Design, the authors lucidly explain, among other ubiquitous but little-understood elements of urban planning, what different colors correspond to: red indicates an electric power line; green marks sewage and drain pipes; orange refers to telecommunications and signal wiring. Mars and Kohlstedt argue these coding systems not only “provide essential information to diggers” but also serve as “ephemeral windows for the rest of us into the complex systems running right beneath our feet.”

The 99% Invisible City is an offshoot of Mars’s popular podcast on design and architecture, 99% Invisible, and many of the book’s entries were originally featured as stories on the podcast, which is now in its 11th year. Both center around the idea that most elements of urban design — whether industrial, consumer, or civic — are hidden in plain view. Prosaic as they may seem, their function and utility can be profound. From imported squirrels to wheelchair ramps, fire escapes to municipal flags, highway numbering systems to wastewater river reversals, the authors find unexpected meaning in everyday infrastructure. “So much of the conversation about design centers on beauty,” Mars and Kohlstedt write, “but the more fascinating stories of the built world are about problem-solving, historical constraints, and human drama.”

As its subtitle suggests, the book models itself as a sort of “field guide,” previously the purview of zoologists and botanists for creating visual taxonomies of the natural world. More recently, the field guide template has been used to help people identify everything from shipping containers to the symbolism of lucid dreams. The 99% Invisible City doesn’t truly emulate a classic field guide. Despite its abundance of hand-drawn illustrations, most aren’t that useful to identifying actual objects in the built environment; this isn’t an urbanist’s equivalent to the Audubon Society Field Guide to North American Mushrooms. But simply invoking the field guide concept allows Mars and Kohlstedt to untether their 100-plus entries from the confines of a more conventional, narrative structure. Entries are organized into sub-sections, many with abstract titles like “Delineations,” “Synanthropes,” and “Hostilities,” but the book is best experienced nonlinearly, by riffling to a random page and acclimating yourself to whichever patch of terrain you land upon. In that sense, The 99% Invisible City feels organized like most cities themselves: dense, seemingly unruly and unpredictable, yet following a distinct logic. The book is driven by the delight of hidden meanings and forgotten histories, especially as they relate to things we encounter every day but rarely stop to contemplate. In one vignette, the authors trace the invention of “Tall Boys,” those noodle-like inflatable figures that have become ubiquitous marketing props. As it turns out, two artists — Trinidad’s Peter Minshall and Israel’s Doron Gazit — created the original versions to tower over the opening ceremonies of the 1996 Olympics in Atlanta before Gazit split off to patent the design for commercial use. Another section offers a mini-history of the traffic light, in which the authors mention that at the corner of Tompkins Street and Milton Avenue in Syracuse’s Tipperary Hill neighborhood, there’s a traffic light that puts the green light on top, a nod to the area’s Irish roots. (A quick Google Street View search confirms that the inverted light is indeed still there.)

One of my favorite entries, “Resourceful Artifice,” delves into a local Long Beach phenomenon I had often noticed when driving along the city’s Ocean Boulevard: four small, manmade isles that dot San Pedro Bay. The Astronaut Islands were built in the 1960s for oil extraction, but unlike the plodding “horse head” pumpjacks in nearby Signal Hill, these derricks boast palm trees, elaborate facades, and neon-colored accent lighting. As it turns out, they were originally dubbed the THUMS Islands, named after the petro-partnership of Texaco, Humble, Union, Mobile, and Shell that ponied up the $10,000,000 bill for “aesthetic mitigation.” Chief architect Joseph Linesch had previously designed faux environments at Disneyland and EPCOT Center. The Astronaut Islands, then, act as a case study for the Southern Californian architectural tradition of incorporating artifice into the “authentic” landscape.

The 99% Invisible City is more than just a series of interesting factoids. At its core, the book is an invitation to contemplate humanity’s efforts to reshape its environs through design, the consequences of which can be far-reaching. In the vignette “Lawn Enforcement,” the American lawn is revealed to have its roots in European aristocratic aspirations that accompanied the original colonists. By turning the lawn into a plebeian entitlement, Americans annually spend billions of dollars, and use trillions of gallons of water, to maintain an entirely cosmetic feature.

One of the great lessons of The 99% Invisible City is that, despite the chaos of urban life, what allows cities to be livable at all is the serendipitous concert of design-oriented projects, policies, and people. The simple act of stepping outside your home and crossing the street is made possible by the countless forces that went into erecting stable buildings, paving sidewalks and roads, and creating traffic systems to manage vehicles and pedestrians — not to mention that uniform color code that safely sorts the city’s subterranean infrastructure. The book is a reminder that the power of invention doesn’t only concern itself with capitalist accumulation. It’s also about trying to improve people’s lives and habitats through the thoughtful union of form and function. The examples are legion but also often liminal; you just need to know how and where to find them.

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In The City Beneath: A Century of Los Angeles Graffiti, anthropologist Susan A. Phillips examines a different set of visual markers that blanket the city: graffiti. Unlike the hidden-but-useful features that The 99% Invisible City surveys, graffiti is mostly seen as the inverse: highly visible but functionally useless, if not an outright nuisance. At worst, graffiti is a sign of urban blight; at best, colorful street art. But for Phillips, graffiti is more than Sharpie tags or Krylon burners — it can also be as simple as a date carved into a rock or names inscribed under a pier, what Phillips describes as “artifact[s] of travel […] [and] stasis.”

Phillips previously authored the 1999 book Wallbangin’: Graffiti and Gangs In L.A., an ethnographic study of West Coast graffiti culture, and The City Beneath is a vital expansion on that scholarship. In compelling and beautiful prose, Phillips investigates the many groups who have quite literally left their mark on Southern California going back over 100 years. Though The City Beneath is partly a history of graffiti practices, it’s better described as a history told through graffiti, with Phillips burrowing into the layered “stratigraphy” of the built environment in order to deepen our understanding of the various communities that moved through the region across the 20th century. At its most basic, tagging public surfaces is a statement that “I was here,” and in seeking to understand who was where, when, and why, Phillips convincingly argues that graffiti constitutes “an alternative written record” that allows one to see “a vision of the city from below” via “notes from the subaltern, the wanderers, the vilified, the vandals, the workers.”

The book begins with train-hopping hobos of the 1910s, who used grease pens to smear their initials beneath bridges, and ends with the much-reviled taggers of the 1990s who traversed the segregated boundaries of South Los Angeles, rattling spray cans in hand. What binds these groups and eras together isn’t aesthetics so much as the absence of authorization. Colored chalk used by a city engineer to mark underground wiring isn’t considered graffiti, but that same chalk used in the same location to write “Jill Loves Jack” might very well be. In other words, what gets labeled as graffiti often turns on whether the writer has permission or not.

Phillips argues that those who create graffiti, in whatever form, are usually those “who have broken the social contract of public space by actually inserting themselves into it.” Case in point: In an early chapter, “Queers,” Phillips visits markings that date back almost 100 years, underneath bridges connecting East Los Angeles and Downtown. The abutments of these bridges became a site where the city’s queer community could make their affections known, including “Mike & Walt,” who began scrawling their paired names as far back as 1928 and possibly through 1934. The simple act of proclaiming queer couplehood on concrete pillars stood in direct defiance against city officials who targeted gay men with arrest, imprisonment, and other pernicious forms of public humiliation. This specific site, underneath the Spring Street Bridge, became a temporary repository for an alternate sexual history of the city. Mike and Walt’s etchings were accompanied by all manners of other graffiti, such as full-scale, nude drawings of heterosexual and same-sex couples along with long, explicit messages written in pencil by “Lee from El Paso.” We don’t, and never will, know much about who created these scant traces; all that remains of Lee is his name, where he came from, and a couple of his sexual preferences.

And yet, these abutment markings affirm the presences of Lee and his peers, and by doing so, Phillips argues their graffiti serves as a “powerful testament to the persistence of queer lives and love in Los Angeles.” They may lack the detail of love letters preserved in an archive, but even in their sparseness and ephemerality, these denotative traces help to complicate and counter the sanitized, official histories of civic life.

Still, Phillips doesn’t seek to romanticize graffiti as some kind of inherently righteous retort by the downtrodden. In “Surfers,” she revisits the beach communities of Santa Monica Bay who, beginning in the late 1950s, began incorporating Nazi symbols — swastikas, iron cross emblems — into scrawls left on surf shacks and pump houses. Some of Phillips’s interviewees read this graffiti with dubious generosity, seeing it as an act of shock value and a reflection of surfing’s ethos of non-conformity rather than antisemitism or racism. One interviewee goes as far to argue that “[if] Malibu was an exclusively white beach community […] how could there be overt racism?” Phillips dismisses this disingenuous line of reasoning, pointing out that communities don’t become exclusively white through random or benevolent occurrence. She does, however, suggest that the incorporation of the swastika in surfing graffiti, with its “hideous symmetry,” captures something about the paradoxical duality of the subculture itself with its “tension of unified opposites” that can “attract the Zen master as well as the racist, which sometimes are one and the same.”

Each chapter in The City Beneath features these kinds of capsule histories and critical readings of different graffiti forms. In “Prisoners,” Phillips travels 200 miles north to Manzanar, to examine the Japanese and English names carved into concrete basis by Japanese Americans incarcerated in internment camps there during World War II. In “Cholos,” she returns to East L.A., arguably the richest neighborhood for graffiti traditions in the region, to look at how Chicano youth of the 1970s developed their own language and style of writing that has since become a ubiquitous visual signifier of “West Coast-ness.” “Grips” reveals how Hollywood studio laborers have secreted messages onto soundstage scaffolding that includes everything from listing production dates to airing complaints about work conditions.

Phillips’s engagement with all of her sites of study is both thoughtful and inspired; it’s also urgent. So much of 20th-century graffiti is on the verge of being permanently erased by 21st-century development. Phillips compares her scholarship to “semiotic triage: to seek out what is left, photograph it, interpret it, and be content with partial knowledge. Meanwhile, the material grows more distant, the people fewer in number, and the messages lost.” While some might celebrate such new developments, as an urban archaeologist Phillips reminds us that objects of today’s derision may very well become tomorrow’s fascinations; more importantly, they are invaluable historical records. “While graffiti in the moment can cause public outcry, give it fifty years, and it becomes interesting,” she writes. “Give it a hundred, or a thousand, and it’s a rare direct link to the past.” Who is to say what today’s graffiti might say about us in eons to come? After all, Phillips reminds us that “[w]ords on the wall [only] remain stagnant until people’s stories begin to inform their interpretation.”

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Oliver Wang is a professor of sociology at California State University, Long Beach. He is the author of Legions of Boom: Filipino American Mobile DJ Crews in the San Francisco Bay Area (2015).