The San Adrian tunnel or Lizarrate represents the most outstanding milestone in the historic inland Basque route of the Way of St. James. It consists of a natural cave carved by water erosion in the rock (called Lizarrate, arguably stemming from "leize arrate", 'the stone gate of the cave') with an opening on either side north and south; it also holds an hermitage inside. The tunnel provides a natural passage dividing the provinces of Gipuzkoa and Álava/Araba (the actual borderline locating at the Alto de la Horca). Incidentally, the Spanish-Basque linguistic boundary of the twentieth century was also established in this area, the next village south, Zalduondo, having been predominantly Spanish speaking during that period.