A patch of phosphorescent green liquid that appeared in Venice’s Grand Canal was caused by the chemical fluorescein.
Environmental officials confirmed the source of the discolouration on Monday after images on social media showed a bright patch of green around the Rialto Bridge, near an embankment lined with restaurants.
Fluorescein – a non-toxic chemical – is primarily used in underwater construction to help identify leaks.
It can also be used in medicine – in the form of eye drops – to identify lesions and foreign bodies, according to the National Institute for Health and Care Excellence.
Despite being non-toxic, the high concentrations of fluorescein found in the canal indicated it probably wasn’t a result of an accident, CNN reported.
Further test results, which could help identify the exact quantity of the substance in the water, are expected later this week.
Italian police launched an investigation into the incident on Sunday.
It followed numerous episodes in Italy, where environmental groups have been targeting monuments with colours and dyes.
Activists used vegetable charcoal to turn the waters of Rome’s Trevi fountain black in a protest against fossil fuels.
However, unlike previous cases, no protest group has come forward yet to claim responsibility for what happened in Venice.