From the outside it looks like a home for a Hobbit or two, but inside security analysts are monitoring banks of screens feeding alerts from hundreds of corporate networks. Welcome to Symantec's European centre of operations, housed in a former nuclear shelter in rural Hampshire.

The site - near Winchester, Hampshire on land formerly used as a reservoir by Southern Water - was chosen for its inherent physical security. Built as a nuclear shelter in the Cold War, the bunker was designed to supply 30 local utilities workers with food, clean air and water for up to 60 days. This group of pre-selected workers were hand-picked to reconnect vital water, gas and electricity supplies across the south of post-apocalypse England.

The sole entrance to the building is via an eight-inch thick steel frame and concrete infill door in the side of a hill. Large blast valves designed to allow a nuclear blast wave to pass through the building can still be seen alongside the entrance. The walls are around 50cm thick and made from reinforced concrete. The concrete on the roof is 2.5 metres thick, reinforced with steel.

The Cold War ended, and the site was taken off the Government's nuclear facility register in 1991. After a spell as a remote alarm monitoring facility (with customers include the Ministry of Defence, gold centres and the Royal family) it became a managed PKI (Public Key Infrastructure) centre, before Symantec took over the bunker in March 2002 and used it to deliver managed security services.

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What is it with anti-virus companies and former nuclear facilities? Kasperky's Moscow HQ is in a building where scientists used to design nuclear weapon's guidance systems. Symantec has gone one step further is hosting a facility in a former nuclear bunker. Is there some kind of "Doomsday nerd" or Dr Strangelove complex going on? We should be told!

Nigel Beighton, Symantec's director of enterprise strategy, EMEA, took our demand in good spirit. Although Symantec wouldn't have built the facility itself it amply suited its purposes, helping it meet stringent security features and standards such as BS7799 that its clients expect, he said. These include redundant, independently routed 54Mbps data pipes, redundant mains power plus a back-up generator as well as an array of physical security features.

Three remote cameras scan the external perimeter. A fourth is trained on the entrance to scan everyone entering and leaving the facility. An access control system guards all areas and rooms in the facility. Its building manager is an affable former submariner who spent 25 years in the Royal Navy and obviously enjoys life under the surface.

The bunker can only be reached up a narrow, single-track lane which sometimes becomes snowed up in the winter. Most of the locals are unaware of its true purpose. Its location on the side of a hill and vents make it look like the home of the Tellytubbies to a casual observer. Symantec staffers tell us that young couples sometimes park in its car park on summer's night unaware that they can be observed from underground from the facility's hidden cameras.