About 7 kilometers away from Ribadesella (in Asturias) is where you'll find the town Cuevas del Agua. This village's special feature is getting there. You have to go through a large cave that the highway passes through. The Cuevona is a 250 meter natural cave. It's very well lit and you can see all of its rock formations. You can leave your car at the entrance and visit or, better yet, you can drive through to the end and park afterwards.