12. Fort Knox, Kentucky, USA

Most people have heard of Fort Knox, but only a select few have been inside. It is known as one of the most heavily guarded places globally because it is home to a large portion of the US gold reserves. The security measures are insane, and none of the staff members are even allowed access to the vault. 

Not only is the building made of concrete-line granite and reinforced steel, but to gain access to the vault, you need to know multiple combinations. The staff members only know one combination, so if they tried to gain access, it would have to be a group effort.