Armed police have been deployed to a stand-off at a former underground nuclear command bunker in Fife.
The bunker was opened as a museum in 1994
A man has shut himself in the bunker, which now operates as a museum, after gaining access at about 0130 BST.
Police do not know if the man is armed or not, but the museum has a wide range of weapons in its displays - including imitation firearms and knives.
Scotland's Secret Bunker is hidden 100ft underground below a farmhouse at Troywood, near Anstruther.
Firearms officers are at the scene but have been unable to trace the man because of the complicated layout of the building, which provides plenty of hiding places.
They are being backed up by trained negotiators.
Chief Superintendent Alan Maich said: "The man broke into the premises and set off the alarm.
"At this time his intentions are unclear. Due to the complex layout of the structure and the fact that he will have access to replica firearms and weapons, caution is being executed."
Mr Maich said the officers at the scene were working "extremely hard".
"There is a CCTV system that has been of limited value," he added.
"The bunker is two floors below ground level and there is no natural light and there are lots of different compartments."
There are replica weapons on display inside thebunker
The man is believed to live locally and arrived at the musem in a mechanical digger.
The former nuclear bunker, which was kept secret for 40 years, is now part of a Cold War museum.
There is 24,000 sq ft of accommodation on two levels, accessed by travelling along a 150m tunnel from the farmhouse.
The bunker opened in 1952 as a Royal Air Force radar station, but was taken over in the 1960s by the Civil Defence Corps.
It was designated as the centre of Scottish government in the event of a nuclear war.
There was space for 300 people to live, work and sleep within the bunker's 15ft-thick reinforced concrete walls.
The bunker opened to the public as a museum in 1994.