Imagine a mountain with a hole through it, and a medieval road running straight through the hole. The tunnel of San Adrián is a natural cave passage near the crest of the Aizkorri range, on the border of Gipuzkoa and Álava, and for centuries it carried one of the main routes between the Castilian plateau and the Basque coast. Pilgrims on the inland Camino de Santiago, merchants, armies and kings all passed through it, and a small chapel was built inside the cave. The old paved road leading up to it survives in sections, worn smooth by centuries of hooves and boots.
The tunnel lies within the Aizkorri-Aratz Natural Park, a range of limestone peaks and beech forests that reach well over a thousand metres. Shepherds still bring their sheep to the high pastures in summer, and the cheese made from their milk, Idiazabal, takes its name from a village not far away; Segura, one of the nearby towns, was a key stop on the road. The forests are carpeted with ferns and moss, and in autumn the beeches turn copper and gold. Mist often swirls around the summits, giving the walk up a slightly enchanted feel.
You cannot ride to the tunnel itself. The usual approach is on foot from the car park at the Otzaurte pass on the road between Zegama and Altsasu, a walk of around an hour each way on a good path, or from the Álava side above Zalduondo. The roads around are a pleasure: winding mountain routes through green valleys and small Basque villages, with frequent changes in weather. Rain is common all year, so expect damp patches, moss and cow muck on the road.
Fuel is available in Altsasu and other valley towns, and the villages have bars and asadores serving grilled meat and pintxos. Alavese villages such as Araia and Agurain sit on the plateau side, where the climate is drier and the landscape opens out into wheat fields. Pack a waterproof, good shoes and a torch for the tunnel, which is short but dark, and take your time on the polished stones of the old road.