A mysterious patch of fluorescent green water that appeared in Venice’s famed Grand Canal Sunday was caused by a chemical commonly used in underwater construction to help identify leaks, environmental authorities say.

The chemical – fluorescein – is non-toxic. It remains unclear 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.

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The verdant blob was first noticed by residents near the Rialto Bridge on Sunday morning local time and grew slowly through the day.

Images showed gondolas, water taxis and water bus boats skimming through the emerald substance.

Read more: yahoo