Storing the art in the quarry during World War Two improved the National Gallery's understanding of preserving art.
Almost 80 years ago Snowdonia prepared to keep a welcome in the hillside for some of the world's most treasured paintings.
Across Europe the advancing Nazis had already looted or destroyed millions of pounds worth of art.
As Allied troops fled Dunkirk, bombs fell on London and a German invasion seemed inevitable, attention turned to how to protect the National Gallery's collection.
Since the beginning of World War Two, the paintings had been stored in various temporary Welsh locations but they were not entirely suitable for long-term use.
In 1940, Winston Churchill famously said of the nation's art treasures: "Hide them in caves and cellars, but not one picture shall leave this island."
https://www.bbc.co.uk/news/uk-wales-48308512" onclick="window.open(this.href);return false;Experts scoured the UK for a hiding place - until they found Manod Quarry in Blaenau Ffestiniog, Gwynedd.
Manod Mountain had been a working quarry for over a century.
Its excavations created a cavernous space at the heart of the mountain, and covered with hundreds of feet of slate and granite it was virtually impregnable to bombing.
Also, its very remoteness made it easier to keep the mission top secret.