The citizens of Venice were shocked to find a patch of fluorescent green water covering the city’s world-famous Grand Canal when they woke up on Sunday morning and the authorities have ordered an investigation to find out the cause of the pollution.

Veneto regional president Luca Zaia wrote on Twitter, “This morning a patch of phosphorescent green liquid appeared in the Grand Canal of Venice, reported by some residents near the Rialto Bridge. The prefect has called an urgent meeting with the police to investigate the origin of the liquid.”

According to a CNN report, the green patch was first noticed at around 9.30 am and grew slowly. City councilman Andrea Pegoraro immediately blamed environmental activists who have been attacking Italian cultural heritage sites in recent months.

However, the Ultima Generazione group, who poured charcoal into the Trevi Fountain in Rome last weekend, told CNN that they did not have any hand in the green water incident.

According to a BBC report, local authorities have collected water samples and opened an urgent investigation. Speculation is rife as to what might have caused the water around the famous Rialto Bridge to change colour with theories ranging from the release of dye to a protest by environmental activists.

Italian media reported that local police were examining CCTV to determine whether the release might have been a stunt to coincide with the Volgalonga regatta taking place this weekend.

Venice’s Grand Canal had earlier experienced a colour change in 1968 when Argentine artist Nicolas Garcia Uriburu dyed the waters of the canal green with a fluorescent dye during the annual Venice Biennale. The move was aimed to highlight environmental issues and the relationship between nature and civilization.