San Adrian

The San Adrian tunnel or Lizarrate is 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 a 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). It is a point of interest on the Basque pilgrimage route of the Way of St. James. The Spanish-Basque linguistic boundary of the twentieth century was established in this area, the next village south, Zalduondo, having been predominantly Spanish speaking during that period. Nowadays many hikers cross the tunnel in order to gain access to the nearby peaks, forests and grazing fields, namely Aratz, Aizkorri and Urbia.

Tunnel data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).