The Pietre Lanciate — the Thrown Stones — are a geological formation of columnar basalt about 3km south of Bolsena, where hexagonal prisms of volcanic rock rise from the ground like a stacked natural pavement. The formation was created when lava cooled slowly and contracted into regular geometric columns, similar to the more famous Giant's Causeway in Ireland.
The site is accessible on foot or by car along a minor road south of town. A short walking trail around the formation explains the geology. It is a good complement to a visit to the castle museum — seeing the geology of the lake firsthand makes the volcanic history tangible.