A 20 metres high vertical wall of slender basalt prisms stands majestically like a pipe organ. On the hill of Mount Cépera (Cuccureddue 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.