A patch of phosphorescent green liquid that appears on the Grand Canal in Venice is caused by the chemical fluorescein.
Environmental officials confirmed the source of the discoloration on Monday after images on social media showed a bright green area around the Rialto Bridge, which is near an embankment lined with restaurants.
Fluorescein – a non-toxic chemical – is primarily used in underwater construction to help identify leaks.
It could also be used in medicine — in the form of eye drops — to identify lesions and foreign objects, according to the National Institute for Health and Care Excellence.
Although non-toxic, the high concentration of luciferin found in the canal suggests it may not have been the result of an accident, according to CNN.
Further test results are expected later this week, which may help determine the exact amount of the substance in the water.
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Italian police launched an investigation into the incident on Sunday.
it followed countless episodes Italyenvironmental groups have been targeting monuments with colors and dyes.
Activists use plant charcoal to close waters Rome’s Trevi Fountain black Protest against fossil fuels.
However, unlike previous cases, no protest group has yet come forward to claim responsibility for what happened in Venice.