The approach of war with Germany in 1939 obliged Clark and his colleagues to consider how to protect the National Gallery's collection from bombing raids. It was agreed that all the works of art must be moved out of central London, where they were acutely vulnerable. One suggestion was to send them to Canada for safekeeping, but by this time the war had started and Clark was worried about the possibility of submarine attacks on the ships taking the collection across the Atlantic; he was not displeased when the prime minister, Winston Churchill, vetoed the idea: "Hide them in caves and cellars, but not one picture shall leave this island." A disused slate mine near Blaenau Ffestiniog in north Wales was chosen as the store. To protect the paintings special storage compartments were constructed, and from careful monitoring of the collection discoveries were made about control of temperature and humidity that benefited its care and display when back in London after the war.