Rupnica, Geological nature reserve in Voćin, Croatia.

Rupnica is an outcrop of columnar-jointed rhyolite southwest of Voćin, on the site of a former quarry. The rock is split into upright columns, mostly four-, five- or six-sided, except in the centre, where they are bent. The lava that formed it cooled quickly, and its composition has not been studied in detail.

Rupnica was declared a protected natural monument on 14 October 1948, the first such geological site in Croatia. The protection covered the former quarry southwest of Voćin, where columnar-jointed rhyolite is exposed. The columns formed as heterogeneous lava cooled, with the first-cooled surface cracking into quadrilaterals, pentagons or hexagons; most stand upright, while those in the centre are bent.

The reserve functions as a place where visitors learn how ancient volcanic forces shaped the land they see today. People come here to explore how rock formations develop and to understand geological processes firsthand.

Rupnica lies southwest of Voćin, within the Papuk Nature Park, at the site of a former quarry. The rock face is reached on foot; the columns stand upright except in the centre, where they are bent. No visitor facilities are recorded at the outcrop itself.

The columnar formations developed through a remarkable natural process where cooling lava fractured into nearly perfect geometric patterns. The regularity of these shapes is so precise that they almost appear human-made, yet they formed entirely through physical forces.