The largest source of obsidian in Sardinia, hidden in the bowels of a volcanic mountain. The green cloak of the Monte Arci massif rears out of the Uras plains, to the far north-east of Campidano, covered by basaltic lava flows that reveal its origins and history over the millennia. The volcanic chimneys culminate in its peaks: the 812 metre high Trebina Longa, 795 metre Trebina Lada and 463 metre Corongiu de Sizoa. Three peaks that bring to mind a tripod.

The area of the Acri regional park (270 squarekilometres) includesland from elevenmunicipalities in the Oristano area - Ales, Marrubiu, Masullas, Morgongiori, Palmas Arborea, Pau, Santa Giusta, Siris, Usellus, Villaurbana and Villaverde.

The obsidian, rarely found in the Mediterranean and ideal for making prehistoric weapons and tools, attracted ancient populations since the 6th millennium BC, who journeyed to Sardinia in order to stock up on the precious mineral. The deposits in PerdasUrias, s’Ennixedduand Su Forru de is Sintzurreddus (at Pau), Roja Cannas (at Masullas) and the Santa Luxia plateau (at Usellus) were points for collecting and working it. In Pau Museum, you can find out about the thousands of years of history created by obsidian. Contemporary sculptures made from huge, black blocks can be found around the town. Museum guides are available to take you on excursions to the quarries. General La Marmora stated that he had never seen such a large quantity of the black, shiny stone anywhere else. While you're in the park, also look out for the Brunk’es’Omu (at Villaverde) nuraghe and the Domu de is Coambus cave sanctuary (at Morgongiori).

Most of the area on the mountain is covered with holm oaks with occasional cork trees, downy oaks and Mediterranean brush. A splendid example of holm oak forest can be found at s'Acqua Frida in the territory of Ales, rich in springs. The woods are inhabited by wild boars, weasels, pine martens and foxes, while roe and fallow deer have been reintroduced. Overhead, you can spot wood-pigeons, finches, jays and hoopoes, while birds of prey nesting here include goshawks, sparrowhawks, lesser kestrels and peregrine falcons.