It's a secret.

It's something that you're not supposed to know until you get to it, a sort of fun surprise as you go through post ranks. As Bfa said, it's a secret.

grumble grumble i thought i might get this answer

Oh well, looks like I'll have to find out by digging through the post count pages.

(It's not one I've already passed, though, is it?)

EDIT: Aha! Found it. Apparently I have indeed already passed it. However, the Emerging Ice Warrior and Ice Warrior Nuva ranks I still have yet to reach. They're still kinda far-ish, though -- so I probably won't bother waiting until those ranks to watch the Doctor Who serial The Ice Warriors like I had planned.