Some TMNT stuff was always meant for grownups.

The Defense Advanced Research Projects Agency (D.A.R.P.A.) is an agency of the United States Department of Defense responsible for the development of new technologies for use by the military. It is headquartered in Arlington, Virginia.

In the Mirage continuity, a fictionalized version of DARPA experimented heavily with mutants, psychic powers, and aliens. One of their agents was Louis Braunze, who in the 1950s was embed with psychic powers. They also are responsible for capturing Raphael and thus act as non-villainous antagonists during the final story arc of Volume 2.

In the volume's issue #2: Winds of Change, DARPA has a secret facility in the mountains "somewhere in the Southwest", which is infiltrated by Baxter Stockman so he can steal a cutting-edge exo-suit.

In issue #8: Face Off, Raphael is captured by DARPA agents who had followed Stockman to New York City, and taken to an apparently different secret facility located on a dry lake bed in Lincoln County, Nevada.