I caught this show from the super structures series on hulu the other night, thought some of you would enjoy seeing it. Discovered in 1936, it was 20 years before the discoverer returned back to the site and another 20 years to begin building it into a mine along with it's access needs. Huge tasks and interesting how they found solutions around those problems.

Grab a drink and sit back, it's a great show.
Super Structures of the World - Grassburg Mine