As the nation marks 60 years since the end of World War II, the role of a remote Snowdonia slate mine in protecting priceless art from the bombers is remembered.
Brick studios and air conditioning were installed in Manod Quarry, near Blaenau Ffestiniog, so the National Gallery's collection could be moved from London.
These included works by artists such as Titian, Michelangelo and Constable.
After the war ended, the 2,000 works were returned intact.
Many of the pieces even arrived home in a better condition, preserved and improved by the humidity and low temperatures inside the mine.
Plans to transfer the art from London led to roads and bridges along the route being altered to keep the works safe.
Emrys Evans was one of the locals to suspect something was afoot
However, although the scheme was meant to be hush-hush, it was difficult to keep the truth from local people.
Emrys Evans was among those to notice something was up when he returned on leave from the Navy.
"A lorry came up the Cwm Road and there were two men in the cab," he told BBC Wales.
"They were both looking very, very hard at me, staring at me, and I thought they were suspicious of me.
"The quarry was working, a number of people were working the quarry - they had eyes and they had ears - and very soon they latched on to what was happening," Mr Evans added.
"It couldn't be kept secret locally, at least I don't think so."
Work is still carried out at the slate mine, which is now known as the Cwt y Bugail quarry.
However, the area where the artwork was housed is no longer safe to enter as the roof has begun to collapse.