Providence 15

Providence 15 is a former providence of the United Bricks that was thought to be destroyed by the Capitol during the first rebellion. It is said to be uninhabitable, the ruins still smoldering from the toxic bombs dropped on it. However, the district is actually fully operational but hidden underground, and in an agreement with the Capitol to remain independent and in exchange, not to start a nuclear war with Panem. Its industries were mining graphite and nuclear science/technology. This is the reason that the Capitol agreed to leave them alone.