A serving Royal Marine has admitted stashing arms and explosives for possible use by dissident republicans.
Ciaran Maxwell, 31, originally from Larne in County Antrim, entered the guilty plea at an Old Bailey hearing in central London.
The Marine was arrested by counter-terrorism officers in August 2016, following the discovery of two arms dumps in woodland near Larne.
The dumps contained land mines, an improvised armour piercing rocket, ammunition and other partly constructed improvised explosive devices and components.
The subsequent examination of those weapons and explosives gave investigators their most significant clues.
They were able to trace some of the explosives back to the British military and eventually to Maxwell.
The serviceman, who was based at 40 Commando's barracks near Taunton in Somerset, was detained three months after the discovery of the second arms dump.
Officers made another significant breakthrough when they discovered a third arms dump near the Marine's home in Devon.
Although he served for years at the heart of one of the UK's most elite units, family and friends said they had no idea Maxwell was living a double life, secretly planning violence against the very state he swore to protect.
The 30-year-old grew up in a predominantly Loyalist community. Although Maxwell is Catholic, those who know him have said they were unaware of any links to republican extremists.
Sources have told Sky News that the Commando was a lone wolf character, with a determination to carry out attacks and link up with other dissident republicans bent on violence.
Maxwell has been remanded in custody to be sentenced at a later date.
The discovery of the Country Antrim arms dumps sparked a major alert and review of security across Northern Ireland, where a small but determined group of dissidents continue to wage a campaign of violence.
Last month, a PSNI police officer was shot and wounded in an attack in Belfast.
Former PSNI Assistant Chief Constable Stephen White told Sky News: "The threat is high. The police and intelligence services are on a high state of alert and that is appropriate to the threat and this weapons find is an indication of the threat we face.
"Whether the weapons themselves are complete, or components, the fact is they prove the capability and the intent of dangerous people, and it's very good news that the weapons were seized."
Mr White added: "I do believe that the security services have been successful in thwarting many of the attempts.
"I think the appetite and political motivation is waning, the support is waning and therefore the supplies are waning, the financed is waning, but there is still some evil people who are getting their hands on weapons."
A review of munitions and weapons security procedures was carried out across the UK's armed forces in the wake of the Royal Marine's arrest.
(c) Sky News 2017