A few days ago I had the privilege of visiting a cave just outside Santander, containing the oldest cave paintings in Europe: La Cueva de El Castillo. It's located a short walk from the village of Puente Viesgo, served by frequent Alsa buses from Santander city centre. The tour guides are professional archeologists, and our tour (consulting of only 3 people) lasted an hour and cost a mere €3!
It was really touching to see an entire wall of hand-prints of all sizes from over 40,000 years ago. The animals were so beautifully and elegantly painted, reminded me of Picasso's quote that "After Altamira all art is decadence" or something similar. Unlike Altamira this cave is still open to the public, but I was informed that the minute the CO2 levels reach a certain level it will be closed.
You will need to join a tour (easily booked online); I was lucky to arrive just after a big school group without booking in advance, but apparently this might be a bit risky over the holidays.
Anyway it was a real treat! Highly recommended for those who can spare the time. A different type of "cathedral" to enjoy on your pilgrimage.