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The Symantec bunker was built by the UK government in the early 1990s to shelter key utilities workers in the event of a nuclear attack and was originally a reservoir. The entrance is set into a hill near Winchester.
For our full feature on Symantec's bunker, click here.For photos of one of the UK's other leading security firms at work, click here for our look inside Sophos' Pentagon.
The airlock door is monitored by a digital camera 24/7. CCTV cameras give a 360 degree view of the outside of the bunker, and have 30 days of digitally recorded back-up.
Blast doors lead into the bunker. They were designed to minimise the shock waves caused by a nuclear explosion. The air inside the bunker is pressurised to 1.5 ppsi greater than the external air pressure, the stop unfiltered air entering the bunker.