A Royal Marine from Northern Ireland has pleaded guilty to offences related to dissident republican terrorism, including bomb-making and storing stolen military weapons.
Ciarán Maxwell, 31, appeared via video link at the Old Bailey in London.
He was arrested in Somerset last August after the discovery of two dissident republican arms dumps near Larne, County Antrim.
He also pleaded guilty to drugs and fraud charges.
Maxwell admitted assisting another to commit acts of terrorism between 2011 and 2016.
He appeared via videolink from Woodhill Prison near Milton Keynes; he spoke only to confirm his name and enter guilty pleas to all the charges.
The marine was remanded in custody, and is to be sentenced at a later date.
Background: David Maxwell, BBC News NI
Ciarán Maxwell was brought up in Larne's Seacourt estate - he began the rigorous training to become a Royal Marine in 2010.
His Facebook page featured pictures of exercises in Britain and abroad.
But the charge suggests he was involved in terrorism from 2011, before he'd even finished that training.
The 31-year-old has a partner and child. What influenced him to turn terrorist may never be known but his case could prompt a review of vetting procedures.
It also raises questions over how he was able to take anti-personnel mines from his base in Somerset to Northern Ireland.
It is understood Ciarán Maxwell has cooperated with police since his arrest and this along with his guilty plea will be taken into account when it comes to sentencing.
The defendant had no previous criminal record but he had made the headlines before.
In 2002 republican paper An Phoblacht reported that he had been the subject of an unprovoked attack by a group of loyalists near his home in Larne.
Maxwell, who is originally from Larne, County Antrim, was based with 40 Commando in Somerset.
According to the charge details, he had a stash of explosives in purpose-built hides in England and Northern Ireland.
Chemicals and maps
He compiled a library of terrorism documents, including instructions on how to make explosives and tactics used by terrorist organisations.
He also had maps, plans and lists of potential targets for a terrorist attack, as well as images of an adapted Police Service of Northern Ireland (PSNI) pass card and a PSNI uniform.
He bought chemicals and components and went on to manufacture explosives and devices, the court heard.
Last March, police said bomb-making parts had been found in barrels hidden in a wooded area in Carnfunnock County Park in County Antrim.
Two months later another "terrorist hide" was found in Capanagh Forest, also near Larne.
Police described it as one of the most significant arms finds in recent years.
Of particular concern to security chiefs was the discovery of military grade anti-personnel mines.
Their serial numbers revealed they had been taken from the Royal Marines base in Somerset where Ciarán Maxwell was based.
Court papers revealed that 12 hides were discovered in total.
Ciarán Maxwell: Charges in brief
- Charge One involved the creation of a library of documents likely to be useful to a person committing or preparing an act of terrorism
- It also related to obtaining chemicals and components for making explosive substances
- It covered the possession of an adapted PSNI (Police Service of Northern Ireland) pass card and items of PSNI uniform
- It related to manufacturing explosive substances and creating and maintaining hides to store bomb components in England and Northern Ireland
- Charge Two related to possession of cannabis with intent to supply
- Charge Three was a fraud charge relating to bank cards
Cdr Dean Haydon of the Metropolitan Police's Counter Terrorism Command said the investigation had resulted in a "significant disruption and protected public safety by removing a large quantity of dangerous material from circulation."
PSNI Assistant Chief Constable Stephen Martin said the case highlighted the benefits of "excellent collaborative working" between the PSNI, the Metropolitan Police and other agencies.
"Working together, we have disrupted the activity of a dangerous individual and removed a very significant threat," he added.
An MOD spokesperson: "We are aware that a member of the armed forces has pleaded guilty to terrorism offences and we will consider the implications very carefully.
"We will continue to fully co-operate with legal proceedings and, with these ongoing, it would be inappropriate to comment further."
Secretary of State for Northern Ireland James Brokenshire said: "While I cannot comment on the particular circumstances of this case as legal proceedings remain to be concluded, it is a reminder of the outstanding contribution made by the police and security services.
"They do vital work to tackle a serious threat and to keep people safe."