http://en.wikipedia.org/wiki/Barbarastollen_underground_archive, http://www.geschichtsspuren.de/artikel/58-ausweichsitze-regierungsbunker/83-barbarastollen-kulturgutschutz.html
Their solution is to put the material to be preserved on microfilm, seal the microfilm into stainless steel drums and put the steel drums in a mine shaft, The big advantage of such an analog solution is that for retrieving/reading the documents you don't need any particular hardware, any light source + magnifying glass will work.
Still, it's only a short-term solution, the films are expected to keep for several hundreds of years (>500) but not for thousands of years.
BTW, Switzerland also has a similar project, also using microfilm stored in a mine.
"I am, therefore I am." -- Akira