As soon as I stepped into this singular space I thought, "Maybe this will stump them."
Not too many clues. We are in Chicago. The structure I'm in is 30 feet tall and 2,000 feet long. It's part of something even larger, or was. You're certainly heard of it, but probably never been there. Few people have, lately. Otherwise, I'll tell you more about it after someone guesses the right answer.
Where is this? As always, the winner receives one of the ever-dwindling stock of this blog's way-cool, ultra-collectible-someday-perhaps poster. Post your guesses in the comments section below.