It may seem a contradiction in terms as these bunkers are no longer secret and indeed many of them are now open to the public as museums.

They date back to the "cold war" and were manned by the Civil Defence Corps, designed to be local command areas in the event of nuclear war and designed to be capable of withstanding an atomic blast and to be self-sufficient with their own power generators and communications. They were usually built outside cities which were considered would have been the main targets though there were some built on the outskirts of cities such as Bristol. When I worked for the Prison Dept Regional HQ in Bristol there was such a one on the complex which ended up being used for records storage but it was fascinating to see the interior frozen in time.

The CDC was disbanded in the late 60s partly because of the politics of the time but also because the bunkers considered to be obsolete in terms of their effectiveness against the megatonnage that could then be delivered.

But those that are open are fascinating places to visit if you get the chance.