The Sala Silver Mine was once Sweden's largest silver mine, but now plays host to the deepest hotel suites in the world. Found 155 ft below ground, and costing $550 a night, the two suites are surrounded by three stone walls and feature little else other than a double bed and a small seating area. The mine also features two dinning rooms that have live entertainment.
Above the mine is just as fun though, featuring high wire adventure tracks but if you prefer to stay underground there are also opportunities to go cave diving in the bracing 35 degree Fahrenheit water. However, the hotel promises "spectacular scenery," for those that choose to take the dive. I personally am a tad claustrophobic, so I'd prefer to stay at the above ground hostels.