"Hide them in caves and cellars, but not one picture shall leave this island," declared Winston Churchill.
It was 1940 and things didn't look good for Britain or the allies.
There were fears London could be carpet-bombed and eventually invaded by Nazi Germany.
Yet the wartime Prime Minister, fearing U-boat attack in the Atlantic, refused to allow the treasures of The National Gallery to be sent to Canada.
"Churchill, I think, knew this was a real propaganda thing," said Suzanne Bosman, author of The National Gallery in Wartime.
"It would give such a bad signal to everyone, you know, rats leaving the sinking ship."
Artworks kept in less than desirable conditions
The gallery's collection, which even back then was worth millions and included works by Van Gogh and Rembrandt, had already been scattered to a series of temporary homes away from London.
But conditions in some places were very poor.
The owner of one Welsh castle housing artworks was reported to regularly get very drunk and at another grand house, wet blankets had to be soaked in a nearby stream and then hung up inside to control the humidity.
"Conservators here at the gallery would just shriek at that suggestion now," laughed archivist Richard Wragg.
Following Churchill's decision, a long-term storage solution had to be found.
A subterranean home
The wild hills in the north of Wales were considered an ideal site — a location both remote yet not too far away from good railway links.
A disused slate mine at Manod, near Blaenau Ffestiniog, was made into a safe, secure and importantly secret, subterranean home for the paintings.
The entrance was enlarged with explosives to get the biggest artworks inside and brick "bungalows" were built underground to create a climate-controlled environment.
"It actually led to new discoveries", Mr Wragg said.
"It had always been assumed that stable conditions were good for paintings but this was the first time it could really be tested.
"The isolation also allowed the works to be thoroughly, meticulously catalogued and examined."
Once a month an artwork was sent back for exhibition in the gallery at Trafalgar Square in central London in order to boost morale in the badly bombed city.
The gallery itself was hit nine times but still attracted many visitors for special events throughout the war, perhaps partly — reports from the time suggest — because of the very high-quality sandwiches on offer.
After the conflict ended in 1945, the Government reserved the Manod site for potential use during the Cold War and only gave it up in the early 1980s.
So, what is the plan if there's another war today?
"Well, there are definitely plans in place for any contingencies," said Ms Bosman.
"Obviously I can't go into any detail about them but there are definitely clear plans."
