Why Did the Water in Venice’s Grand Canal Turn Bright Green?

Venetians awoke Sunday to find that a section of water in the city’s famed Grand Canal had mysteriously become neon green.

Now, Italian authorities say they know what caused the chartreuse hue around the popular Rialto Bridge—but they still don’t know who is responsible.

Testing showed the presence of fluorescein, an environmentally harmless dye that can be used to locate blood at crime scenes and diagnose eye injuries. It can also help determine whether sewer systems are working properly, according to a statement from the regional environmental agency for Veneto, the state that encompasses Venice.

The chemical is typically not dangerous to humans, but it can cause rare problems for some individuals. Officials said they did not detect any substances in the water that would give rise to environmental concerns, per the statement.

#Venezia zonaRialto#presenza colorazioneverdeanomala#Arpav sul posto per accertamenti ambientali con @vigilidelfuoco e @Pol_Municipale pic.twitter.com/7v71QodfNA

— ARPA VENETO (@arpaveneto) May 28, 2023

Built on a series of small islands, Venice is connected by canals and footpaths—not roads. Lined with historic buildings, the Grand Canal is one of the main thoroughfares through the city. As such, it’s often brimming with boats transporting residents and tourists alike. The Rialto Bridge, which spans the Grand Canal, is the oldest of the four bridges that allow people to cross the high-traffic waterway.

Authorities weren’t surprised to learn fluorescein had caused the water’s abnormal coloring, as similar incidents have occurred with the chemical in the past, per the statement. However, given the quantity of dye in the water—at least two pounds—they are concerned that someone, or a group of people, might have dumped it intentionally. The usual amount is just a small spoonful, per the BBC’s Laura Gozzi.

Officials took samples at different depths in the water to determine how much fluorescein sunk to the bottom of the canal. They are also testing water from neighboring canals to see how far the dye spread, per the statement.

It could take a few days for the fluorescein to dissolve in the sunlight, but it depends on how much is in the water.

Stamattina nel #CanalGrande di #Venezia è apparsa una chiazza di liquido verde fosforescente, segnalata da alcuni residenti all’altezza del Ponte di Rialto.
Il prefetto ha convocato una riunione urgente con le forze di polizia per approfondire l’origine del liquido. pic.twitter.com/te0JVCfodQ

— Luca Zaia (@zaiapresidente) May 28, 2023

This is not the first time Venice’s Grand Canal has turned green. On June 19, 1968, Argentinian ecological artist Nicolás García Uriburu performed a similar stunt as part of a work of art. He dumped fluorescein into the Grand Canal to “bring attention to the relationship between nature and civilization and to promote ecological consciousness as a critical part of culture,” according to the Metropolitan Museum of Art. Afterward, he kept bottles of the bright green canal water as a “testament to his artistic intervention” and took photos and videos of the scene, per the museum. Two years later, he dyed several other prominent rivers green—the East River in New York, the Seine in Paris and the Rio de la Plata in Buenos Aires—and repeated the process once more in Venice’s Grand Canal. Then, he made six screenprints, collectively called Portfolio (Manifesto), as part of the project, per the museum.

So far, no one has come forward to claim responsibility for the most recent dye job, so it’s unclear if they were following in Uriburu’s footsteps with their actions. However, authorities are concerned about copycats following the latest perpetrator’s lead. Given the visibility of Venice and of this incident, the region’s president, Luca Zaia, wrote on Facebook that this event could motivate similar actions by people who want attention.

“We must protect the city, its monuments, everyone’s right to enjoy its historical treasures free of the marks left by rowdy individuals,” wrote Zaia, per the New York Times’ Jesus Jiménez.

Police are reviewing surveillance footage in the area and interviewing anyone who may have witnessed the person or people who tinted the water green, report CNN’s Barbie Latza Nadeau and Sophie Tanno. Though a city councilman blamed the environmental activist group Ultima Generazione, its members denied any involvement to CNN. In late May, members of that group poured diluted charcoal into Rome’s Trevi Fountain to raise awareness about human-caused climate change.

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Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.