Something's in the water of Italy's famed Grand Canal in Venice.
Venetians woke up to a mysterious patch of fluorescent green in the central waterway on Sunday. The verdant stretch hugged an embankment near the Rialto Bridge, and appeared to spread as the day went on, based on images posted to social media.
Police are investigating the origin of the liquid after residents reported the spectacle to authorities on Sunday morning, Luca Zaia, the governor of the Veneto region, said on Twitter.
Italy's fire and rescue agency said it's working with local environmental officials to identify the substance from collected samples of the water.
A range of theories have surfaced online, with some speculating algae growth and others pointing the finger at environmental activists.
City councilman Andrea Pegoraro blamed climate activists, CNN reported, who have been targeting Italian cultural sites in recent months.
The group Ultima Generazione, which has poured charcoal into fountains in Rome at least three times since April in protest, told the network that the green coloring "wasn't us."
The timing of the emerald patch also aroused suspicion. The Vogalonga rowing regatta, in which amateur crews finish along the Grand Canal, took place over the weekend. The amateur boat event aims to celebrate the Venetian lagoon environment as well as to protest wave motion from excessive motorboat traffic that riles the city.
For many, the bright green waters call to mind a 1968 exploit by eco-artist Nicolás García Uriburu. According to the Metropolitan Museum of Art, Uriburu colored the waters of the Venice Grand Canal that year with a neon-green dye called Fluorescein to promote environmental awareness.
