I'm willing to bet that the big door at the bottom is access to the other silos.

I'm also quite sure that Holston is probably dead. Along with everyone else, that went outside.

I had this thought last night, after seeing the finale. What if the other silos are open to each other, and only Silo 18 (I'm guessing it the number based on the key to the mainframe having number 18 on it) is closed off / isolated? Maybe because of it being the origin of the rebellion? As some sort of sanction. Or maybe to contain The Syndrome - whatever that even is.

I know it's far fetched. But who knows? (Other than the millions of people who've read the books, of cause ;-))