A patch of bright green that appeared in Venice’s Grand Canal, on Sunday, May 28, was caused by the chemical fluorescein, environmental officials confirmed on Monday.

The nontoxic chemical is used in underwater construction to help identify leaks, but the high concentrations found in the Grand Canal indicate its presence probably wasn’t an accident, CNN reported, citing environmental officials.

This footage by Vigili del Fuoco, Italy’s fire and rescue agency, shows the bright green waters around the Rialto Bridge in Venice. Credit: Vigili del Fuoco via Storyful