It is difficult to write about the new MI5 building in Holywood when so many facts are unknown. But here are a few facts that are known. In 1994, three months before the first IRA ceasefire, the cream of Northern Ireland's intelligence establishment fitted comfortably into the back of a helicopter. When that helicopter crashed into the Mull of Kintyre it killed 25 senior army, police and MI5 officers.
The new MI5 headquarters has visible office accommodation for an estimated 400 people. In 1994, with a war still raging and a peace process looming, MI5 was involved in operations against loyalists and republicans. Its new remit covers only dissident republicans. Is it really credible that a building of this size has been constructed even partly, let alone primarily, as a response to the threat from the micro-IRAs? Far greater and more numerous threats were handled for decades from small offices inside Castlereagh and rusty Nissan huts behind Thiepval Barracks.
MI5 might well have a dissident desk in its new location but a few desks could easily be the size of it. The Holywood headquarters is also known to have an underground section. Building below ground is three times more expensive than building above ground, even for conventional structures.
The surrounding Palace Barracks site comprises army housing which the Ministry of Defence notoriously regards as a low priority. Were these houses really spared at such expense for an underground car-park? Published estimates of the cost of the building range from £20 million to £100 million but the NIO has admitted contributing an unknown sum from its security budget. Yet for two years the NIO could not find the cash for Northern Ireland's long-overdue and politically sensitive new policing college. Luckily, political sensitivities have been conveniently assuaged.
Both Sinn Féin and the British government now discuss MI5's role in Northern Ireland in terms of 'civic policing'. This phrase is so absurd that it is unlikely to have occurred to both sides independently. There has never been any suggestion of a role for MI5 in routine law enforcement. Holywood was not chosen for MI5's largest base of operations outside London so that bored spooks could set up speed-traps on the Bangor dual-carriageway. However, the site is right beside an airport, a port, a railway, the hub of the motorway network and the seat of Northern Ireland's civil administration. Can this be a coincidence?
Here are a few more facts which may cast some light on the mystery. Eliza Manningham-Buller, the outgoing director general of MI5, says that a chemical, biological or radiological attack on London is "only a matter of time". MI5 has been closely involved in drafting Operation Sassoon, the evacuation plan for London in the event of such an attack.
The plan calls for 300,000 people to leave the city, but only because this is the largest number that can be evacuated without the resulting chaos costing more lives than it saves. Everyone else will be "invacuated" ie told to stay at home and tape their windows shut.
As a result, even a small dirty bomb will paralyse London for weeks and there is no prospect of the British government invacuating itself for the duration.
Most official buildings in London are connected by a network of tunnels 30 metres below ground leading to war-rooms and 'citadels' from which the country can be run in an emergency. In 1955, scientists from the Ministry of Defence's Porton Down research centre released harmless spores into a section of the tunnels beneath Whitehall to see if they would spread. The classified results, leaked to Channel 4 News in 2001, found that the spores had "readily penetrated" supposedly secure areas, leading to "extensive contamination of many Whitehall buildings". Further experiments concluded that the rest of the tunnel network was equally useless.
So here is an explanation that might make sense of all these facts. Belfast has been chosen as an alternative site for national MI5 operations and related government functions in the event of a catastrophic terrorist attack. It was chosen because it is the UK's most remote and ethnically homogeneous major city, making it easy to secure against follow-up attacks. Mindful of local sensitivities, the Sinn Féin leadership was kept informed and language was agreed to neutralise the issue, leaving an uninformed SDLP standing on the Bangor dual-carriageway waving placards. Unionism neither knows nor cares.
How ironic that London is threatening Belfast with a Plan B.
Belfast is London's Plan B.