On some occasions, the need of maintaining a high level of security for certain infrastructures is the main reason to put those infrastructures underground.
The principal security advantage for underground facilities is that access points are generally limited and easily secured. Another advantage is that of secrecy: underground facilities are normally hidden from the public view.
These are some of the reasons why in most countries all facilities which are very strategic to the well-being and security of the country are normally located underground. This can be the case for facilities devoted to sensitive research, highly strategic military installations, prisons, bank reserves, informatics servers, etc.
This fact has even fostered imagination of the general public, leading to “legends” of secret underground facilities where the army keeps important secrets related to extra-terrestrial activity, for instance.
As an example, recently a swiss company has proposed to store in a former underground military fortification a number of computer servers, so that information is kept secured and safe against all kind of events: nuclear crisis, theft, etc.