The word “graffiti” usually conjures images of people with spray cans illegally making murals or jotting down tags using colorful paints. A lot artistic interventions use other tools and materials, though, subverting expectations and working in (literal and legal) gray areas to create works without leaving a conventional trace. Consider, for instance, the massive deep sea monsters, jungle predators and swamp creatures of Russian illustrator Nikita Golubev that lurk in the grimy shadows on the sides and backs of trucks.

Along similar lines, this series of skulls by artist Orion was made by scrubbing car exhaust from an active tunnel. For those looking to deter street art and artists, subtractive interventions like these can be tricky to pin down. After all, Golubev and Orion are simply cleaning vehicles or public surfaces, albeit very selectively. In many cases, the end result is actually further cleaning — art like this often pushes municipalities to send out teams that then wash off entire areas to make them look consistent again.

This artistic approach draws different reactions depending on the scale and situation. Take snow calligraphy, for instance — few people seem to mind a nice message traced into the hood of their car. Artist Faust notes that virtually “everyone has an affinity for writing in the snow as a child,” so perhaps it’s also something people feel they can relate to on a more experiential level.

Reverse graffiti not only provokes different response—it also spans a variety of materials and methods and can work with greenery as well as it does with snow or grime. Some moss artists work additively, creating mixtures to apply to surfaces and thus encouraging moss to follow particular grown patterns to produce an specific result. Others, however, actively remove moss to create desired words, patterns and illustrations.

Stefaan de Croock falls into the latter category, using power-washing tools to create cityscapes and other whimsical scenes on surfaces previously covered in layers of moss. As with a lot of reverse graffiti projects, his pieces are generally temporary — the moss simply grows back in to fill the voids over time.

SpY’s work in Besancon, France, combines elements of reverse graffiti and tree sculpting. Turning topiary approaches into a mural-making technique, he shaped vines into a circular work of wall art using an elevated work platform, trimming his way toward a perfect circle.

Light art and projection bombing are even more temporary and generally even less invasive than reverse graffiti. Lighting machines aimed at buildings to create patterns or spell out messages can be targeted and disabled if they persist, but ephemeral light painting like the above work by Lichtfaktor are deployed quickly using glowsticks or other portable devices and have to be captured on camera to work, making them brilliantly elusive.

Meanwhile, if light art is about brief visibility, then hydrophobic spray art is about lasting invisibility. Both are made to be seen and not seen in particular ways, but the latter has a key ingredient that determines when a design or artwork is visible: wetness.

When the hydrophobic NeverWet spray came out, it promised to waterproof everything, but some users found they had mixed results in applying it to things like clothing and touchscreens – it discolored shoes and left a sticky film on devices. Then someone thought to create a stencil and tag sidewalks with the stuff and a new type of visible-when-wet graffiti was born. One can, of course, free-spray with it as well, but stencils help make the outcome more controlled and predictable.

The same basic artistic idea has been applied in other contexts by designers, too — selective hydrophobia can be incorporated into brick pavers or concrete sidewalks, for instance, to create patterns that change with the weather. Clever urbanists might consider ways to integrate useful messages or wayfinding elements into such projects, like arrows pointing to nearest sources of shelter in a storm. Similarly, messages that appear on surfaces that change color with temperature could be used to guide people on particularly hot or cold days.

Practical designs aside, artists continue to experiment outside of traditional material palettes with works that raise questions about visibility and invisibility. There is fun to be found at the intersections of these ideas, embodied in projects like this one by 3D artist Milane Ramsi, who combined different approaches into a single installation — a concrete pillar appears to vanish while simultaneously revealing three-dimensional lettering more reminiscent of conventional graffiti.

Additive or subtractive, vandalism has its grey areas, but what about the seemingly more straightforward removal of graffiti? Here again there are shades of gray. Some artists, like Mathieu Tremblin, paint a surface clean then write over graffiti with their own work — in his case: humorously replacing loose tags with digital-style tag clouds. That, clearly, is of a kind with what was underneath; in places where tagging is illegal, replacement tags are too.

Then there is the saga of the infamous Banksy and the so-called Gray Ghost, an anti-street artist who leaves signature splotches of gray in the wake of his graffiti removals. Some argue the Ghost’s work vandalizes art — others say it is itself art. Legally, like Banksy’s illegal murals, both artists work in a similar space, though in Banksy’s case building owners often go to great lengths to preserve his art, in part for its monetary value.

Exceptions aside, municipal laws are usually clear on painted public art and the goal is generally total graffiti erasure, though it doesn’t always work out that way. This kind of official cleanup usually draws binary responses, viewed by some as a welcome fix and by others as an act of defacement. One award-winning film, however, argues for a third point of view: graffiti removal as the ultimate next step in the progression of modern art. Despite its semi-satirical intent, The Subconscious Art of Graffiti Removal raises provocative questions about what constitutes street artwork in the gray areas of additive and subtractive graffiti.

An oft overlooked wonder of the urban wilderness, the common pigeon is much more than it appears, with a rich history of aristocratic breeding dating back centuries.

Photographer and native New Yorker Andrew Garn has spent years documenting these creaturing and collecting them into a book titled The New York Pigeon: Behind the Feathers to share their “personalities, expressiveness, glorious feather iridescence, and deeply hued eyes.”

Birds that used to be exchanged between the wealthy as gifts have grown to have different associations, and created a whole industry of pigeon defense systems for urban architecture.

The goal of Garn’s work is, in part, to cast them in a different light (both literally and otherwise), showing the elegance of these creatures when taken out of everyday contexts where some find them to be annoying pests.

Some shots put subjects against dark backgrounds, focusing the viewer necessarily on the birds themselves. Others put them in their urban contexts, highlighting them in flight or perched on windows.

For more on the history of this adaptive series, check out this episode of 99% Invisible. And for more photographs of pigeons at all stages of their lives, see Garn’s website or buy the book.

3D graffiti artist Milane Ramsi has combined two challenging types of urban art into a single installation, making a concrete pillar appear to vanish while producing three-dimensional lettering.

The installation wraps a concrete support post for a highway overpass in Karlovy Vary, a city in the Czech Republic. The work spells the artist’s name in reverse, appearing to pierce the pillar thanks to a combination of colors overlaid on a simulated background. The purple gives a general shape to the work while the yellow adds apparent depth and dimension.

His other works range from tags to 3D-looking works that appear to pop off the wall, but this particular piece combines elements that span graffiti types and styles.

In invisibility aspect is reminiscent of another clever work of roadside art, a wrapped tree designed to look like it has been cut across the middle, leaving the top half portion of the trunk and branches to seemingly float on air. And below: Tree Line by photographer Zander Olsen.

Unseen above the restored cast-iron facade of this 1857 residential structure in Tribeca, a stunning modern loft extends the building’s livable space, its form responding directly to lines of sight.

Designed by WORKac and known as the Stealth Building, the addition had to work with the existing facade — to preserve its appearance, rendering it invisible became the obvious approach.

In order to accomplish this clever act of concealment, all angles of view and shadows cast had to be accounted for. In turn, these informed the shape of the angular extension. Tracing lines of visibility across facade features of adjacent buildings, the designers derived a roof line that would work with the project goals.

All of this naturally shaped the interior spaces as well, creating dramatic diagonal lines that carry through the various rooms.

The fifth-floor pop-up features sleeping quarters, entertaining and dining spaces, as well as a secluded terrace tucked behind the pediment to provide privacy for residents. The former elevator bulkhead has been converted to contain a hut tub servicing the deck as well.

Preserving the natural beauty of a UNESCO-protected valley in the Jordanian desert, this underground architectural complex is designed burrow and branch out beneath the surface.

This conceptual proposal by Rasem Kamal of Oppenheim Architecture features nearly 2,000,000-square-foot network of spaces containing a train station, museum and hotel. The idea aims to bring services to the area with minimal disruption to the natural beauty of the sand and sandstone of the Wadi Rum. In turn, these additions would set the stage for a residential development.

The so-called Excavated Sanctuaries bring in natural light from above via limited openings while using the thermal mass of the desert to regulate interior temperatures below ground. Inspired by the nests and warrens of ants and other subsurface creatures, the layouts are at once complex but organized, buildings semi-autonomous but interconnected. The renderings have a Dali-esque quality, highlighting the surreal as well as the serene beauty of simple architecture in a desert setting.

Aside from its contextual response, there is a deeper thesis here as well: good architecture does not have to stand out – at times, it is called upon to blend in or be invisible.

“Lately, a great many prominent architectural practices have been focusing on developing dynamic forms, new building materials, sophisticated details and tectonics as well, while only the minority of these contribute to their internal spaces,” explains Kamal.

“Consequently, this thesis aimed to flip the relationship between the explicit and implicit, by diminishing the power of external form along with exploiting all the previous efforts that were used for it to subtract spaces where we will live, experience and enjoy.”