About a kilometer and a half from the historic center of Viggianello, you find the Sorgente del Mercure (Mercure Wellspring), the birthplace of the Lao river, also known as the Mercure river in the Basilicata region.

The wellspring has a flow of 2,000 liters per second and a temperature of a cool nine degrees Celsius (48 degrees Fahrenheit). Thanks to a settling tank built in the 1920s, the crystal clear water basin flows to a hydroelectric power plant through a covered tunnel. There are few fountains where visitors can take fresh water. 

A local story holds that the town's name, Viggianello, was inspired by an event at Sorgente del Mercure. The story goes that a queen was passing through the Mercure Valley and wanted to quench her thirst in the clear waters of the spring. When she bent down, she dropped her wedding ring in the water. The queen then ordered her guards to search the length and breadth of the wellspring in the hope of finding it. When hopes were almost lost, a soldier saw a glint in the crystalline waters and exclaimed: "Vidi Anellum, mia Regina" ("I saw Anellum, my Queen"). Delighted to have found her ring, the queen baptized the place with the name of Vidianello.