Drawings by Leonardo da Vinci on show at the National Library of Wales were previously stored there during World War II, researchers have found.

...officials discovered that the drawings had been placed there for safety in a specially built secret cave of masterpieces.

...

Within hours of the declaration of war 70 years ago in September 1939, collections from many of Britain's cultural institutions were crated up and sent by train to Aberystwyth, she added.

Twenty-five containers arrived from the British Museum alone, and some institutions even sent their cultural experts to Aberystwyth with the collections.

To ensure the safety of such precious artefacts, the national library took the decision to carve out an underground cave to house some of the evacuated material.

But by May 1946, all the material had been returned by rail to their collective institutions and the library cave's has remained empty ever since.

If I remember rightly (and I've got an account of this somewhere) Aberystwyth was the first choice, but there were concerns about the suitability of the location (both physical and geographical) and eventually Manod was chosen in preference.

Grahami

y 1940 the risk of bombing in Wales had greatly increased and it was felt that the National Gallery's pictures were no longer safe in the public buildings and private houses in which they were being stored. Ian Rawlins, the Gallery's scientific adviser, was asked to find alternative accommodation and on 17 September discovered Manod Quarry, a slate mine in the mountains above the village of Ffestiniog. Two hundred feet of solid rock covered Manod, access to its caves was through a long tunnel, it had electricity, water, and reasonable road access - it seemed perfect for the storage of the paintings.

Preparations to house the pictures at Manod took almost a year. Transporting them from their scattered locations began on 12 August 1941 and was completed on 18 September. It was carried out by the London Midland Scottish Railway and the Great Western Railway, who had also moved the collection from London.