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.
Military - Bases, Sea - Military - Submarines
Links: en.wikipedia.org
By: kjfitz
http://www.kockums.se/en/products-services/marine-services/musko-facilities/