“This morning a patch of phosphorescent green liquid appeared in the Grand Canal of Venice, reported by some residents near the Rialto Bridge. The prefect has called an urgent meeting with the police to investigate the origin of the liquid,” Veneto regional president Luca Zaia wrote on Twitter.

The local prefect spokesperson told CNN that they immediately took water samples, reviewed CCTV surveillance tape and asked local gondolier pilots and boat drivers if they saw anything suspicions, before calling an emergency meeting to investigate the cause of the green water, noting that no environmental group had claimed responsibility.

The verdant blob was first noticed around 9:30 a.m. CET (3.30aET) and grew slowly, according to multiple images posted on social media, which showed gondolas, water taxis and water bus boats skimming through the emerald substance.

City councilman Andrea Pegoraro immediately blamed environmental activists who have been attacking Italian cultural heritage sites in recent months.

The group Ultima Generazione, which poured charcoal into the Trevi Fountain in Rome last weekend, told CNN when asked if they were behind the green water, “It wasn’t us.”

Italy’s fire brigade tweeted that they were assisting with providing “samples and technical assistance” to the ARPA Veneto, the regional agency that oversees the environmental state of the Grand Canal, which are “conducting analysis to establish the nature of the substance in the water.”

Various theories surfaced online, including that it could be algae or a substance illegally dispersed in the canal.

This is not the first time Venice’s Grand Canal has experienced a color alteration.

In 1968 Argentine artist Nicolás García Uriburu dyed the waters of the canal green with a fluorescent dye called Fluorescein, during the annual Venice Biennale. The move was designed to bring attention to ecological issues and the relationship between nature and civilization.

The curious coloring comes as the city is celebrating the Vogalonga boat event, created to combat wave motion and to restore Venetian traditions and help spread attention for the environment and nature as well as the architecture Biennale, which opened last weekend.