A 20 metres high vertical wall of slender basalt prisms stands majestically like a pipe organ. On the hill of Mount Cépera (Cuccureddu ‘e Zéppara in campidanese dialect) in Medio Campidano at an altitude of 167 metres is the town of Guspini, home to a very rare geological formation: a small volcanic cone that has been declared a Natural Monument and World Heritage Site by the European Union. It was formed by the slow and gradual cooling of the lava remaining inside the inner cone of the volcano which forced perfect vertical fissures into prismatic columns in polygonal sections.

In the 19^th century, part of the hill had been used as a quarry. When it was declared a National Monument, the quarry was closed and is now a lovely verdant wall of great scientific interest. Columnar basalts are a typical example of an effusive eruption, a phenomenon marked by the slow rising of magma into a narrow area and subjected to intense pressure. The lava was pressed upwards out of a magma chamber connected to the Mount Arcuentu and the Monte Arci complex into a sort of secondary volcanic mouth. There is no lack on the island of similar basaltic formations, but the ones in Guspini, which you can see following a dedicated path, are extraordinary both for the particular conformations and the precision of the fissures, and thus a rare example not only here in Italy and in Europe as a whole. They can be found only in the Veneto and Sicily, and, in the rest of Europe, only in Iceland. Patagonia and India are the only other places that can boast similar phenomena.