One thing to note is that the UK had a huge number (over 1500) small underground observation posts operated by the Royal Observer Corps - in the case of war these were to monitor nuclear explosions and fall-out:

https://en.wikipedia.org/wiki/Royal_Observer_Corps_Monitorin...

This short film from 1971 that includes scenes filmed in observation posts and a central bunker:

https://www.youtube.com/watch?v=o3y0WmONP4E

http://www.subbrit.org.uk/rsg/sites/s/skendleby/ was turned into a datacentre now run by GCI but previously known as SmartBunker.

Circa 10 years ago I was involved in a project helping transition it from 80s phone exchange into a modern facility built to a good spec. Getting connectivity out there was painful and expensive; at one point it was backhauled wirelessly to Skegness!

Barnton Quarry bunker in Edinburgh (about 4 km west of where I live) is being renovated and will hopefully be open soon:

http://www.secretscotland.org.uk/index.php/Secrets/BarntonQu...

This bunker would have been the Regional Seat of Government for Scotland:

https://en.wikipedia.org/wiki/Regional_Seat_of_Government

Given that its location and purpose wasn't a secret I imagine it would have been blown to bits in any conflict.

I was under the impression that the RSGs were secret until 1963 when an anti nuke campaign group exposed them?

But then, hey, Britain. We have a load of secrets that aren't technically secret but we'd rather you didn't know and would take a dim view of you trying to find out. Especially true during the Cold War (see e.g. Backbone, etc).

Good point - I'm not sure when Barnton stopped being the RSG - I had assumed it was well after '63.

There was a new RSG type bunker at Cultybraggan near Comrie but, with impeccable timing, that wasn't completed until 1990!

Outside of Houston, Texas, this former private bunker (http://www.houstonbunker.com/about/) was turned into a secure data warehouse. I believe Continental Airlines used to use it for their data.

If this kind of thing interests you and you are in the south of England (or even Brittany in France) you should definitely check out the WW2 German bunkers in the Channel Islands. They are easy to reach by sea or air, especially during summer. I'm in Jersey and have access to a few large ones, contact info is on my profile page. Here is some background info if you are unfamiliar: https://en.wikipedia.org/wiki/German_occupation_of_the_Chann...

We regularly have people visit from all the way up and down Hitler's 'Atlantic Wall' and they really like the number and condition of the bunkers here.

Paris has some crazy catacombs. I think Google Streetview should just go down and index it. Then there'll be plenty of people doing ghost hunting on their computer screen.

GPS doesn't work underground.

A few WiFi beacons and some inertial guidance, done.

See also:

http://www.derelictplaces.co.uk