The abandoned factory in Barakaldo

On the outskirts of Barakaldo there's an abandoned factory where you can do it in the car nearby, or go inside the factory, where there are several interesting areas, though it's pretty dirty (that's abandoned buildings for you). On the fourth floor there's a room with an old bed, and the more daring can go up to the roof. It's a bit tricky to get there, but it's worth the effort, as you'll have total peace and quiet and great views.