District 13!

The History of District 13
The Capitol has told everybody that District 13 was one of the districts in the revolution that mined grafite and was obliterated in the Capitol, yet in Catching Fire, Katniss meets a woman named Bonnie who is fleeing from District 8 to District 13. She says that it still exists, and used to be producing nuclear weaponry.

Bonnie's Proof

Bonnie believes that District 13 exists because every time that the Capitol shows footage of District 13, it's the same footage, even when it is supposed to be live. She believes this because there is always the same wing of the same mockingjay at the same time in the upper right corner of the screen.