Locals and tourists traversing Venice’s Grand Canal were greeted by an eerie sight on Sunday morning, with a patch of water near the famed Rialto Bridge appearing to have turned phosphorescent green. “The prefect has called an urgent meeting with the police to investigate the origin of the liquid,” tweeted Veneto regional president Luca Zaia. A spokesperson for the prefect told CNN that authorities had taken water samples, reviewed nearby surveillance tapes, and interviewed possible witnesses. Local daily La Nuova Venezia reported that police were probing whether the green coloring may have been the result of a stunt by climate activists, according to The Guardian. No environmental group immediately claimed responsibility in the matter, the prefect’s spokesperson said. The Grand Canal’s waters were previously dyed green in 1968 by neo avant-garde artist Nicolás Garcia Uriburu in a statement on water pollution at that year’s Biennale.