There is a Titan silo, in Denver, that's free to tour. I think it even has a pool at the bottom.
Actually there are 3 in Aurora and 1 out by Deer Field as well as others. They were pretty cool back in the early 80's when I would "tour" them. Paid for a few shop bills hauling diamond plate out of them. Oh and the very bottom of them smelled of rotten eggs.