A place where I used to work. Now decommissioned, so I can talk about it a bit.
When you're stuck deep underground for long periods, you lose track of things like whether it's day or night outside, or if long enough, what season it is. So such places aren't the bare and spartan locations usually associated with secret squirrel stuff in the publuc imagination. Secure complexes can be quite large and airy. Apart from the tastefully decorated pillars and low ceilings.
Here's a shot of the rec room during decommissioning.

Entry to the bunker complex was via the sub sub basement of a secure office building some distance away. To get to this stage, you already had to have gone through the Maxwell Smart stuff, going through what seemed to be just another cupboard door in a corridor in the basement. The armed guards etc were a long way behind you by this stage.
This was the entry foyer to the tunnel leading to the complex.

The view from the tunnel entrance looking out, showing the "nondescript cupboard door" leading to the real world outside, and just another corridor in the sub sub basement. There wasn't much traffic, so if someone entered the door and didn exit for a few days, weeks or months it wasn't noticeable. Security through obscurity, or just plain being nondescript and unremarkable.
Apart from the emergency exit, with its long, long spiral staircase leading up to what looked on the surface like a utilities box about the size of a Tardis, * this was the only access point.

One of the many equipment rooms, with some of the gear too obsolete to salvage.

The view in the corridor, just outside where I used to work, oddly enough, doing stuff. There were many such corridors leading to various sections.

One of the security doors between sections. Everything highly compartmentalised, Need To Know regardless of clearance.

Another one, again with reminders of the outside world.

* - which caused some mirth during evacuation drills. Bystanders would see a door in a utilities box far away from any building open, and a hundred people would emerge from it....
