A haul of arms discovered in Lurgan may have been there for some time, sources have said.
Workmen are initially thought to have stumbled across some of the weaponry.
During two days of follow-up searches by officers using specialist equipment, several other small dumps were discovered in the vicinity of a derelict property on the outskirts of the town.
The material is believed to belong to dissident republicans.
Lurgan is considered a republican stronghold and both the group calling itself the 'IRA' and the older Continuity IRA have a presence in the area.
It is third significant arms find since the start of the year.
In May, bomb components were discovered at Capanagh forest near Larne and in March, arms were also discovered at Carnfunnock country park close to the town.
Police were unable to say how long the latest arms haul may have been there.
However, republican sources have said the majority of the find was older material that had been concealed in the area for some considerable time.
Detective Superintendent Karen Baxter said one of the items was a "fully constructed explosive device" and that mass murder had potentially been avoided.
"These items were in the advanced stages of readiness and some were ready to be used," she said.
Other material found included home-made explosives, improvised mortars and launchers, pipe bombs and three firearms.
A stun gun and timing devices were also among the haul.
Policing Board chair Anne Connolly said it was a "great result for the PSNI to find these firearms/munitions and take them away from those who would have used them to cause harm in our community".
Ulster Unionist MLA Doug Beattie also said it was a "major success and a significant blow against republican terrorism".
"Some of these devices were in the advanced stages of construction. Without a shadow of a doubt, their removal has saved individuals from death or serious injury", he added.
Sinn Féin MLA Catherine Seeley said: "The fact that those would try to roll back the progress we have made as a society, regardless of who they claim to represent, will not be able to use these weapons is to be welcomed.
"I'm glad these materials have been discovered and taken out of circulation and cannot be used to endanger life.".
