Italian authorities are investigating what could have caused Venice's Grand Canal to turn fluorescent green over the weekend.
The bright green hue suddenly appeared in the water on Sunday, sending locals and tourists alike to question what was behind the strange color.
"It's green all in this area," NBC News correspondent Keir Simmons' boat taxi driver, Nicola, said underneath the Rialto Bridge on TODAY. "Like the Hulk (in) 'The Avengers.'"
Police launched an investigation, and officials said the green color was caused by a harmless chemical called fluorescein.
"Fluorescein is a tracer in powder form that dilutes in water and is used, for example, to detect the proper functioning of a sewage system," the Veneto region's agency for the protection of the environment said in a statement.
The agency added the substance is not dangerous for the aquatic environment.
While the green tint had mostly disappeared by Monday morning, investigators are still searching for the cause.
"The investigation is looking everywhere," Michele di Bari, the head of Venice's prefecture, said through a translator on TODAY. "All hypotheses are open."
The canal had been turned green once before — in 1968, Argentine artist Nicolás García Uriburu dyed the waters green to spread ecological awareness.
Other climate change activists turned the waters of Rome's Trevi Fountain black earlier this month, and threw gold paint at a statue in Milan earlier this year.
"I think it was the activists," Nicola told Simmons.
An American tourist celebrating his 10th wedding anniversary noted the similar color to the Chicago River in March.
"They do this kind of thing on Chicago on St. Patrick's Day," he said.
