People constantly discuss the issue of secure systems and often get it wrong.
Consider a server that is secure against network attacks by being physically separated from any networks. Plus, all the machines that can access it are similarly secured. The OS is secure and the users are all vetted and non-hostile. This network resides on the floor of an office building, with armed security guards and multiple layers of physical access protection. The network has it's own UPS's and generators, and all the machines are in heavy hermetically sealed containers that are EMP hardened.