Authorities in Venice have confirmed the cause of its famed Grand Canal turning fluorescent green on Sunday.
The mysterious patch of bright green water that appeared and puzzled onlookers was triggered by a chemical commonly used in underwater construction to help identify leaks, environmental authorities have said.
The chemical - fluorescein - is non-toxic.
It remains unclear as to how the substance ended up in the canal, but the Regional Agency for the Environment in Venice (ARPAV) said given the volume released it was unlikely to be an accident.
Italy's Fire and Rescue Service had teams investigating what the substance was after it was first noticed by residents near the Rialto Bridge in the morning and then grew slowly throughout the day.
Photos showed gondolas, water taxis and water bus boats skimming through the emerald substance.
No group has claimed responsibility for the act.
Local police are investigating a number of leads, including environmental activism, a spokesperson for the Venice Police told CNN.
Further test results are expected later this week, which could help identify the exact quantity of the substance in the water.
Luca Zaia, the president of the region of Venice, warned that environmental activists may carry out copycat acts.
This is not the first time Venice’s Grand Canal has turned green.
In 1968, Argentine artist Nicolás García 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.
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