The Oñati-Arrikrutz Caves are a karst system formed in Upper Jurassic–Lower Cretaceous limestones and dolomites, dating back between 154 and 96 million years. Located 400 meters above sea level in the Aizkorri massif, they constitute the largest cave system in Gipuzkoa and the only cave in the province open to the public with regular guided tours. The Gesaltza-Arrikrutz karst system features 14 km of interconnected galleries distributed across six superimposed karst levels, although the tour is limited exclusively to Gallery 53, a route ranging from 500 meters to 1 kilometer in length. During the tour, equipped with helmets and headlamps, visitors can see stalactites, slow-growing stalagmites, underground rivers, and small waterfalls. The main attraction is the complete skeleton of a 35,000-year-old cave lion—the only one of its kind on the Iberian Peninsula—alongside an impressive site containing cave bear remains. Displays provide information about the historical presence of hyenas, woolly rhinoceroses, and cave deer that inhabited the cave as a prehistoric refuge. The cave is part of the Aizkorri-Aratz Natural Park and is located near the Arantzazu Sanctuary, between the villages of Arantzazu and Araotz in the municipality of Oñati. Its geological significance is recognized in the Inventory of Sites of Geological Interest in the Basque Country. The interior temperature remains constant at 10 degrees Celsius regardless of the time of year, so visitors should wear appropriate warm clothing.
