My description is just based off a blurb that is equally vague in ILM: Into the Digital Realm. At some point, the Fox archives outgrew their own compound so they had to start storing some of them off-site. They selected a former salt mine in Kansas and turned it into a film vault.
I have no idea how much of the Star Wars material, if any, Fox kept. Lucasfilm seems to have all the negatives in their own archives now.