The Citadel, technically known as the United Nations Continuity-of-Species Bunker (UNCSB) is a massive structure built under the Canadian Shield - one of the densest geological structures on Earth. Begun in 1960, the Citadel was designed as a response to the near-extinction of humanity during the Human-Sen'kaal War; that is, as a practically impenetrable last refuge for humanity in the event of a total defeat of human military forces. The Citadel is also noteworthy for being the largest, longest-running, and most expensive construction project in human history (As of 2013, it is still only 89% finished.)
While much of the data regarding the exact composition (and even the location) of the Citadel is highly classified, what is known is that it is located over three miles underground - completely invisible to ground-penetrating radar. It is in the form of a massive, gently sloping dome that is over three miles in diameter. The reinforced concrete walls of the sructure are estimated to be anywhere from forty to 600 feet thick.