There is an extinct volcanic crater in the Arizona desert that Turrell has been working on since 1974. To me it seems like he's trying to create a cosmic and spiritual experience for the pilgrim who makes the trek out to the desert. In a lot of religions, devotees have to make a long trip through barren land to get to an amazing mecca. Roden Crater feels like that kind of place. I can't wait until the second phase is complete and it opens up to the public.
Check out this link for more info on Roden Crater.