The Musko submarine base in Sweden is inside a mountain. The thumbnail and URL are focused on one of the two large entrances. There is a smaller one just to the south and another big one way around the peninsula to the NE.
This is actually a bombproof shipyard with drydock capabilities constructed by the Swedish Navy during the Cold War era. By Googling Musko underground drydock several excellent images turn up, this link has an interesting diagram of the facility as well:
http://www.kockums.se/en/products-services/marine-services/musko-facilities/