The Tunnel route runs fo 200 km. From Irún on the French border to Santo Domingo de la Calzada on the Camino Francés (the main spine route of the network of pilgrimage routes which comprise the Camino de Santiago or Way of St. James)
It gets its name from St. Adrian's Tunnel high up in the Sierra de Urquilla. The tunnel is a natural one, but it has a long history of use by pilgrims and merchants, probably going back to before the Roman conquest. The route is very varied, with some sections of ascent and descent in mountains, and other sections in agricultural land in the valleys. These pages will give you a good idea of what to expect.