Cuevas de Oñati-Arrikrutz

Located 5.5km southwest of Oñati, this cavern system has numerous slow-growing stalagmites and stalactites. Visits are by guided tour and take you through one 500m gallery of this vast 15km-long network, where skeletons of woolly rhinoceroses, Irish elks and cave hyenas have been discovered. Along the way, you'll see replicas of the now-extinct creatures that lived 35,000 years ago. Book tours through Oñati's tourist office before heading out.