Underground is the one in a series of DLC planned for The Division. It features new gear sets, gear trading, new game modes, and more. Underground releases June 28th on Xbox One and PC, and August 2nd on PS4.
Very little is known about this DLC piece yet. Rumors tied to an Australian gaming company suggest that the DLC expands the Dark Zone to cover the entire map, although whether or not the user must opt in to this has yet to be seen or discussed by Massive.