Monuments Men Inside the Mines - Archives of American Art Blog it’s fair to wonder why so many mines were used as repositories.
They were ideal for storing art for various reasons. At depths of up to
800 meters underground, the mines provided shelter from bombing; they
were sprawling networks of tunnels capable of storing the large caches
of artwork; and their interiors provided stable conditions safe from the
vicissitudes of weather above ground.
The inside of the mines was cool, dark and dry. According to an Archives of American Art oral history interview with Monuments Man and conservator George Stout,
who supervised the recovery of art from Merkers and Ransbach
repositories, the mines were relatively dry with a low average relative
humidity of 40%.