Fort Knox
noun /ˌfɔːt ˈnɒks/
  /ˌfɔːrt ˈnɑːks/
Idioms Idioms 
be like/as safe as Fort Knox 
- (of a building) to be strongly built, often with many locks, strong doors, guards, etc., so that it is difficult for people to enter and the things kept there are safe
- This home of yours is like Fort Knox.