The following paraphrased information is taken from:
Sonderauftrag Linz, Ernst Kubin, Vienna, 1989.
About 5000 paintings were acquired by Hitler from various sources for the Sonderauftrag (Special Mission), which was the cover for establishing a huge museum in Linz.
They were first stored in the cellar of the Führer headquarters in Munich, and because of shortage of space were also stored in castles and monasteries. However, because of the development of the war with increasing bomb attacks since 1943, they were no longer secure and were therfore sent to be housed in salt mines deep in the Austrian Alps. The mines were ideal for the art work because of their constant atmospheric conditions and secure location.
The empty tunnels in the saltmine at Altaussee in the eastern Austrian Alps were lined with wood and equipped with electricity. Although art from different parts of Austria had already been stored there, in the winter of 1943-44 the collection for the Linzer Museum was ordered to be brought there by Hitler.
Later, because of the expected end of the war, and the allies advancing, Hitler ordered the entrances of the tunnels blocked, but not to destroy the art.
Then, in April 1945 further boxes labelled, "Marble, Keep Upright", were delivered to the mine. But because of the weight and the shape of the boxes which were to be brought to the inside of the mountain, the workmen are suspicious of the contents.
It was discovered that these boxes were delivered by the provincial head who wanted to blow up the whole salt mine in order that , "the objects of art should by no means come into the Jewish hands."
The head foreman of the mine and the chief technician then discuss what to do in order to save the art, and their workplace. They manage to reach one of the most influencial security personnel of this region in the declining Third Reich. He gives a contradictory order to prevent the blowing up of the mountain. But the order is not accepted and the danger remains. The authority who delivered the bombs, threatens to come himself with hand-grenades and do it himself if the bombs are removed. However, the workmen do remove the bombs and put them in the countryside. Fearing looting after the war, they then dynamite the entrances of the tunnels.
The allies advance and the war is over.
May 11, 1945 is the first time the allies try to get into the mines which also hid Hitlers personal diaries. On June 14 all tunnels were opened, and on June 17 the first 5 trucks loaded with the 5000 pieces of art leave for the collecting point in Munich where the art was held until it could be relocated and distributed.