For days, Venetians have wondered what caused a swath of their city’s famous Grand Canal to turn bright green. On Monday, the authorities had an answer: Test samples of the water confirmed that the canal’s bright new hue was caused by fluorescein, a chemical often used to find leaks during underwater construction.

The spectacular transformation of a stretch of Venice's Grand Canal to fluorescent green was due to fluorescein, a non-toxic substance used for testing wastewater networks, local authorities said.