What's with District 13?

In the 2nd book, Catching Fire, Katniss meets Twill and Bonnie, who are two people from District 8 that fled in the uprising. They have the Mockingjay symbol ,on the cracker they show Katniss, which represents the rebellion in Panem. Twill explains how they never actually show District 13, they Capitol just broadcasts an old clip that shows ruins and rubble, from when the Capitol destroyed it years ago. What is the Capitol hiding? Is it still rubble, or has the Capitol done something top secret that they don't want to show Panem's residents? What do you think? Leave a comment below with what you think the Capitol has done with District 13!