What Happend To District 13?

Back in the 'Dark Days' (The Dark Days is a term commonly used to describe the collapse of the first rebellion in Panem, which occurred 74 years before the first book) , District 13 initially led the rebellion against the Capitol. However, the Capitol took action against this, and informed all of the citizens in Panem that they had destroyed District 13 in penance for their uprising. However, it's revealed in Mockingjay that the Capitol did not in fact destroy District 13. They permitted them to live, in secret, underground, as long as they promised not to try and rebel again; however, they still told the citizens of Panem that it had been destroyed.