Some call it “the dark side”, others “the dirty war”. It is a place of life and death.
Over the years different names have emerged at different times and for different reasons — the names of some of those who crossed the war lines from the republican and loyalist sides to work for the police, the Army and intelligence services.
Loyalists Brian Nelson, William Stobie, Ken Barrett and John White. Republicans Denis Donaldson and Freddie Scappaticci are just some of those names.
Some were executed, others are in hiding or in jail.
I have been to a scene where bodies were dumped after IRA interrogations — and I have been in the room as the IRA gave detailed explanations for the executions.
To be found out — to be revealed as a traitor — means you die, usually shot in the head, your body dumped to be found and seen, this the final humiliation for the collaborator.
This is what I mean by life and death — and this is why this newspaper has not revealed the names given to us in the anonymous letters.
The author of those letters believes a number of recent attacks could have been prevented — the killings of the two Sappers at Massereene Barracks, the murder of Constable Stephen Carroll, and the bomb attack on Constable Peadar Heffron.
It is easy to say. No information is provided to support that assertion.
Intelligence is never a perfect picture. There are always gaps — missing pieces.
Some would suggest that the RUC did it better than the PSNI, that after the Patten reforms too many experienced officers have left police ranks.
Again, it is something that is easily said.
When the RUC had the intelligence role the IRA drove two bombs into Thiepval Barracks — the Army’s Northern Ireland Headquarters — drove mortar bombs into position for an attack on Downing Street, and came close to killing Margaret Thatcher during her party conference in 1984.
It does not matter how many agents you run there will always be blindspots, many things that are not seen or heard.
There has been a massive shake-up of agent running. New guidelines put in place, many CHIS dumped because of their involvement in crime.
Some argue the intelligence-gathering system has been damaged.
But there is proof that dissident attacks are still being prevented — one example took place in Garrison in Fermanagh recently.
The full extent of the use of agents is one of the hidden secrets of the war.
It is a can of worms waiting to spill out.
In 2002 the IRA breached Special Branch security — and was behind a robbery at the Castlereagh police offices in which the codenames of agents and the names of their handlers were stolen.
The use of agents and intelligence is part and parcel of every war.
But in Northern Ireland the puppets and the strings became a tangled mess.
There have been many twists and turns in this intelligence war.
And every time that information is leaked lives are being put at risk.