Officially, it doesn't exist. It's 10,000 square feet, boasts a massive underground section, and could cost up to £100 million but MI5 won't even admit it's building a new headquarters beside Palace Barracks, Holywood, a few miles outside Belfast.
For nationalists, such expansion is hardly a sign the British are leaving the North. But there are other reasons to worry. All the gains of recent years, in terms of accountability and transparency regarding policing, are in jeopardy.
Next year, MI5 will take over intelligence-gathering from the Police Service of Northern Ireland (PSNI). The SDLP and Sinn Féin are challenging this move in the ongoing post-St Andrews negotiations, but no-one believes the British will reverse it.
The darkest, most undemocratic forces in the state are in the ascendancy. "Allowing MI5 to have a lead role in intelligence is like appointing Herod as children's commissioner," says SDLP leader, Mark Durkan.
Paul O'Connor of the Pat Finucane Centre in Derry is equally disturbed: "MI5 was aware Pat Finucane was to be murdered but didn't intervene. MI5 allowed the 1998 Omagh bombing to proceed, it destroyed covert recordings in shoot-to-kill cases, it ran agents involved in multiple murders.
"The Bloody Sunday tribunal, the commission of inquiry into the Dublin, Monaghan and Dundalk bombings, and the Stevens and Stalker investigations, all faced major difficulties when requiring co-operation from MI5.
"MI5 officers have been involved in interrogations at Bagram Air Base in Afghanistan and in Guantanamo Bay. It's unbelievable that an increased role is being planned for such people."
Sinn Féin says: "We reject any role for MI5 in Ireland or in civic policing. We want to see democratically accountable civic policing and we will continue to work until we achieve this."
MI5's hands are steeped in blood. In his 2003 report, Sir John Stevens concluded that British agent, Brian Nelson, colluded with the UDA in 15 murders and played a role in 14 attempted killings and 62 murder conspiracies. He provided UDA gunmen with the names of Catholics who had no IRA connections.
According to former British intelligence officer, Martin Ingram, MI5 was also aware of Nelson's links with Lebanese and South African gun-runners. Nelson would meet MI5 representatives in London before flying to Johannesburg to meet arms' dealers. His weapons helped the UDA and UVF intensify their violence in the run-up to 1994, increasing pressure on the IRA to call a ceasefire, MI5's prized objective.
Unlike republican and loyalist paramilitaries, MI5 has never admitted, let alone apologised, for any wrong-doing during the conflict. "An organisation which can't account for what it did in the past, should have no role in the future," says Durkan.
By giving MI5 primacy in intelligence-gathering, the British are ensuring that those making the crucial security decisions in the North escape the transparency and equality requirements which apply to the PSNI.
The practice of 50-50 Catholic-Protestant recruitment has increased Catholic representation in the PSNI to over 20%; it will be over 30% by 2010. Not only has MI5 no 50:50 recruitment policy, it won't even reveal the religious break-down of its existing workforce.
It won't say what arrangements are in place for monitoring Orange Order or Freemason membership among staff. Director-general, Dame Eliza Manningham Buller, is the only senior MI5 figure whose identity is publicly known. The names of MI5's head in the North, and other senior figures, remain secret.
Sources say that, as part of its expansion in the North the new HQs can house up to 400 people MI5 is recruiting ex-RUC Special Branch officers, who left the PSNI because of their opposition to reform.
The PSNI is subject to rigorous and robust accountability. Chief Constable, Sir Hugh Orde, is answerable to the Policing Board, made up of local politicians and independent figures. The PSNI is also subject to investigation by Police Ombudsman, Nuala O'Loan, who has the power to arrest and question officers, and seize documents.
O'Loan has no remit over MI5. Complaints about MI5 can be made to an 'investigatory powers tribunal'. It's a whitewash: of 380 complaints made from 2000-2004, not one was upheld.
MI5 runs rings around the parliamentary intelligence and security committee which is meant to oversee it, and has escaped with lies about events before the 7/7 London bombings.
MI5's role in the Omagh bomb was equally murky. FBI and MI5 agent, Dave Rupert, who infiltrated the Real IRA, told his handlers that dissident republicans were planning to bomb Omagh or Derry. MI5 never passed this information onto the RUC. The families only learnt about it eight years after the atrocity in which 29 people were killed.
Had police known Omagh was a dissident target, they might have responded more efficiently to the August 15 bomb warnings, the families believe. They have sought a meeting with Dame Eliza to find out why MI5 withheld the information. She has so far refused to meet them. They also wrote to British Home Secretary, John Reid, who is responsible for MI5. The Home Office 'lost' the letter.
Michael Gallagher, whose son Aidan was killed in the bomb, says: "We've been trying to meet Dame Eliza since March. We have met prime ministers and taoisigh but this woman is so powerful, we have no access to her."
Nearly a fifth of MI5's £1.3 billion annual budget is spent on the North. Its website material suggests tackling loyalist violence is hardly a top priority. Loyalist vigilante groups were "originally formed in the 1960s and 70s to defend their neighbourhoods against republican violence", MI5 says. This version of history is completely at odds with the Malvern Street shootings, the burning of Bombay Street, and the murder of countless Catholics.
Top priority is given to dissident republicans who "present a serious threat to British interests" and "aspire to attack targets in Britain". MI5 details dissident republican attacks in which no-one has died but fails to mention numerous loyalist murders, including that of Catholic teenager, Michael McIlveen, in Ballymena in May.
The switch from PSNI to MI5 primacy in intelligence-gathering in the North is linked to the 2002 Castlereagh break-in. After the Provos stole files from PSNI headquarters, the British government asked former Northern Ireland Office permanent secretary, Sir John Chilcott, to conduct a review.
His report, which has never been published, is understood to have recommended MI5's new, expanded role. As the Castlereagh break-in ultimately benefited MI5, conspiracy theorists wonder if it could have been planned or suggested by British agents within IRA ranks.
The Sunday Tribune has been told the PSNI will continue to run loyalist paramilitary agents, but that MI5 is anxious to maintain control of republican operatives.
Annexe E of the St Andrews Agreement says MI5 will "continue to run directly a small number of agents who are authorised to obtain information in the interests of national security as distinct from countering criminality".
It is understood this includes agents of political influence, as Denis Donaldson was, within Sinn Féin. This would save blushes all round, if agents were unmasked after Sinn Féin signs up to policing. If the agents were PSNI-run, Sinn Féin representatives on the Policing Board would look silly. If they were controlled by MI5, Sinn Féin could shrug its shoulders.
PSNI Assistant Chief Constable, Peter Sheridan, has developed five principles to increase MI5 accountability and intelligence-sharing. Nuala O'Loan is in discussions with the organisation over her office's access to it. But history suggests that, whatever mechanisms are created, MI5 will just work around them.