The world is full of icons that warn us to be afraid — to stay away from this or not do that. And many of these are easy to understand because they represent something recognizable, like a fire, or a person slipping on a wet floor. But some concepts are hard to communicate visually, especially in a way that will work for generations to come. 99% Invisible teamed up with Vox to bring you this video about the challenges designers face in developing warning symbols that last:

Take the Jolly Roger, for instance. It was once one of the most feared symbols in the world. It represented death, pirates, and poison. But today? A skull-and-crossbones is associated more with treasure, blockbuster movies, or Halloween than actual danger. Designing something that retains its meaning over time is a surprisingly difficult.

Back in the early 20th century, there was an urgent need for a new kind of warning symbol. At the time, there was no universal standard for communicating the presence of dangerous biological materials.

Laboratories at the US Army used an inverted blue triangle. Those at the Navy used a pink rectangle. The Universal Postal Convention used a white staff-and-snake on a violet background. The lack of consistency put people at risk of exposure and infection.

So in 1966, a group of engineers and designers at Dow Chemical set out to create the best possible icon for biohazardous materials.

They laid out six design criteria. The solution had to be:

Striking in form in order to draw immediate attention Unique and unambiguous to avoid confusion with other symbols Quickly recognizable and easily recalled Easily stenciled Symmetrical, in order to appear identical from all angles Acceptable to groups of varying ethnic backgrounds

Those criteria ruled out simple shapes in use at the time, like those from the Navy and Army, and ambiguous symbols, like the snake-and-staff, which has various medicinal associations.

Charles Baldwin, an environmental health engineer at Dow, sought to develop a visual icon fitting the criteria, “that was memorable but meaningless … so we could educate people as to what it means.”

He and his team showed a set of 24 symbols to 300 people from 25 American cities. There were six newly-designed biohazard markers, and 18 common symbols — things like Mr. Peanut, the Texaco star, the Shell Oil symbol, the Red Cross, even a swastika.

Participants were asked to guess the meaning of each one, which was used to assign each one a “meaningfulness score.” A week later, the same participants were shown those original 24 symbols, plus 36 more and asked to identify which symbols they remembered seeing before.

Among the six competing biohazard designs, one stood out. It scored the highest in memorability, but the lowest in meaningfulness. So it was unforgettable, but also a totally blank slate for designers who wanted to give it meaning. It became a national standard.

It’s easy to overlook how much visual communication work this kind of symbol is doing. The design is simple — you only need a straightedge and a compass to recreate it. And unlike most other hazard symbols, it doesn’t reference a visible object or idea. Yet it has remained iconic for decades, helping people recognize serious dangers that may remain a threat for thousands of years to come. And that raises the question: could the meaning of symbols like this one stand the test of time?

Few people have pondered that question quite like Gregory Benford, a physicist and science fiction author. In the 1990s, he was invited to work on the Waste Isolation Pilot Project, or WIPP, a massive storage site for radioactive waste in the southeastern plains of New Mexico, organized by the US Department of Energy.

Benford was brought in to help calculate the probability that someone or something would intrude on the site for as long as it remains dangerous — approximately the next 10,000 years. It turns out, few things (outside of organized religions and ritualized traditions) last that long. A symbol like the Jolly Roger, for instance, wouldn’t work for WIPP — people might not understand it, or think it marked buried treasure.

Since the 1970s, engineers, anthropologists, physicists, and behavioral scientists have proposed different design solutions to the problem.

One idea was to add context to the symbol. By illustrating cause and effect in a three-part cartoon, designers could communicate the danger even if the symbol lost its meaning. But this assumed people would read left to right and understand causality between frames.

So other designers started to focus on creating a warning without inscribed communication, by altering the shape of the location itself. They drew up spike fields, forbidding blocks, giant pyramids — designs capitalizing on natural human instincts of fear and discomfort. Even then, though, designers couldn’t be sure whether these structures would be perceived as terrifying or fascinating.

So without symbols, or basic illustrations, or physical structures, how can a designer effectively communicate a warning? That’s where the more philosophical design solutions come in.

In 1984, the German Journal of Semiotics published a series of solutions from various scholars. Linguist Thomas Sebeok, for instance, proposed creating an atomic priesthood, where an exclusive political group would use its own rituals and myths to preserve knowledge of radioactive areas, like a church. Philosophers Françoise Bastide and Paolo Fabbri proposed to genetically engineer bioluminescent cats that would glow in the presence of radioactivity. By creating songs and traditions about the danger of glowing cats, the warning could last as long as the oldest relics of civilization we have: culture.

There’s no definitive solution for warning people far into the future.

But designing clear, inclusive symbols will continue to be a fundamental part of how we keep people safe, at least in the present. We will change, and so will the ways we communicate visually, and our warning symbols will have to change along with us.

Even now, the power the biohazard symbol once had to inspire awe and fear has begun to diminish. Today, it appears on everyday clothing and products, slowly becoming more ordinary than extraordinary.

Check out more collaborative design videos from 99pi and Vox: