When a stretch of Venice’s Grand Canal started to turn fluorescent green on Sunday, many people’s first assumption was that it was a stunt carried out by environmental protestors.

Activists from Last Generation have staged a long series of protests across Italy to call attention to the climate crisis, including dyeing the water in Rome’s Trevi Fountain and the Fountain of the Four Rivers in Piazza Navona black.

But this time, the group denied their involvement. “It wasn’t us,” they told police.

READ ALSO: Climate activists turn Rome’s Trevi Fountain water black

As it turns out, the canal’s transfiguration had a more banal cause: the green colour was due to fluorescein, a non-toxic substance used for testing wastewater networks, local authorities said on Monday.

Una chiazza fluorescente ha colorato di verde parte del Canal Grande e della laguna a #Venezia: prelievi e assistenza tecnica da parte dei #vigilidelfuoco agli operatori dell’@arpaveneto che stanno conducendo analisi per stabilire la natura della sostanza in acqua#28maggio pic.twitter.com/vgmUeRthKK

— Vigili del Fuoco (@vigilidelfuoco) May 28, 2023

Analysis showed “the presence of fluorescein in samples taken”, said the the Regional Agency for Environmental Prevention and Protection of Veneto (Arpav).

The results “have not shown the presence of toxic elements in the samples analysed”, the statement said, without specifying the origin of the substance.

Scatto…guardate di che colore è oggi l'acqua a Venezia 🤩🤩🤩 pic.twitter.com/wDlKe3Wcpt

— Monica 🍀⚘️🌍📚🐞🎧 (@Monica09058845) May 28, 2023

Where the dye came from and how it got into the canal in the first place is still something of a mystery.

It is not the first time the Grand Canal has turned green.

In 1968, Argentine artist Nicolas Garcia Uriburu dyed the waters of Venice’s Grand Canal green with a fluorescent dye during the 34th Venice Biennale in a stunt to promote ecological awareness.