The liquid is less ominous than originally assumed.
When Venice's famous Grand Canal suddenly turned bright green at the weekend, police and locals were understandably concerned.
Gondoliers could be seen punting through the phosphorescent waters while tourists took photographs of the green patch, from the Rialto Bridge up and along part of the canal.
Some pointed the finger at climate activists Last Generation. They did turn Rome's Trevi Fountain black, after all. But they told police they were not responsible.
But local authorities have now discovered that the liquid was fluorescein, a non-toxic substance used for testing wastewater networks.
Where did the bright green liquid come from?
The results "have not shown the presence of toxic elements in the samples analysed", the statement said.
Where the dye came from and how it got into the canal in the first place is still a mystery.
Has the Grand Canal ever been green before?
As it turns out, it's not the first time the canal has been green: in 1968, Argentine artist Nicolás García Uriburu put fluorescein in the water to bring attention to the relationship between nature and civilisation during the 34th Venice Biennale. The low tide made the dye gradually disappear over the course of one day.
Watch the video above to see the bright green Grand Canal.