A number of military citadels are known to have been constructed underground in central London, dating mostly from the Second World War and the Cold War. Unlike traditional above-ground citadels, these sites are primarily secure centres for defence coordination.

A large network of tunnels exists below London for a variety of communications, civil defence and military purposes; however, it is unclear how these tunnels, and the various facilities linked to them, fit together, if at all. Even the number and nature of these facilities is unclear; only a few have been officially admitted to.

The Ministry of Defence (MOD) Main Building in Whitehall was outfitted with two bunkers, known as the North and South Citadels, when first built. The site of the South Citadel was later used for an improved "Defence Crisis Management Centre" bunker. The bunker is named after the ancient Greek poet Pindar whose house was the only one left standing in Thebes following the city's destruction in 335 BC.

Planning for the Pindar bunker began in 1979, with ministerial approval being granted in November 1982 and construction starting in 1984. Originally due to become operational in 1989 or 1990, the bunker instead became operational in December 1992. The bunker cost £126.3 million, £66.3 million of which was spent on the civil engineering element; computer equipment was much more expensive to install than originally estimated as there was very little physical access to what was a confined site.

Pindar has two floors; the lower floor contains the Ministry of Defence's Joint Operations Centre (previously situated on the fifth floor of the MOD Main Building), and the upper floor consists of Government Emergency Rooms (comprising the Prime Minister, Secretaries of State, the Cabinet Secretary, and some Permanent Secretaries), an element of the Joint Intelligence Organisation, and a telecommunications secretariat and a Cabinet Office Communications Centre (COMCEN) element. Onsite facilities include stores for items ranging from CBRN equipment to personal hygiene products, a television studio and broadcast centre, a decontamination suite, briefing and conference rooms, a "crisis control room", and a medical bay. Pindar can house "an absolute maximum" of four hundred personnel and provides protection against the blast, radiation, and electromagnetic pulse effects of "all but a direct hit or very near miss" by nuclear weapons; it further provides protection against conventional bombing, sabotage, biological and chemical attack, and flooding, though an 1987 assessment determined that it could be potentially overwhelmed by newer precision-guided munitions. The bunker was intended to be capable of thirty days' independent operation, seven of which could be spent in a sealed condition in case of persistent chemicals or nuclear fallout.

Pindar is connected to Downing Street and the Cabinet Office by a tunnel under Whitehall; the tunnel predated the bunker and was already used as a conduit between the Cabinet Office and the MOD Main Building, with Downing Street access being added during Pindar's construction. The tunnel can be used by government ministers to enter Pindar without risking the press attention (and subsequent damage to national morale) that would ensue if the bunker was openly entered and, as was the case when the bunker was used for meetings on the 1999 NATO bombing campaign in Yugoslavia, without the risk of encountering hostile demonstrations. When answering written questions about Pindar, which included a question on the extent of lift and staircase access to the bunker and on whether there was any connection to transport systems, then-Armed Forces Minister Jeremy Hanley would say only that there were "sufficient means of access and egress" and denied that the bunker was connected to any transport system; he also said that there were means of leaving Pindar should the MOD Main Building collapse on top of it, but did not state the details of these.

Although Pindar is not open to the public, it has had some public exposure. Between September 2006 and April 2007, the British photographer David Moore carried out an extensive photographic survey of an underground facility that was widely believed (and strongly hinted) to be Pindar, with both Moore and the Ministry of Defence stating in later years that Pindar was indeed the facility depicted in the photographs. The photographs were published as The Last Things in 2008 as well as being exhibited in 2008 and 2009.

In addition to the bunker under the Main Building, it was intended that a reserve location be established at the site of the Kingsway telephone exchange in High Holborn; while a bunker was indeed established in the eastmost tunnels it is unclear whether this was connected to Pindar, and in any case it was abandoned in 1996. A further change in intentions relates to the "COBR" Cabinet Office Briefing Rooms; while it was intended that Pindar would replace these, the idea appears to have been abandoned and the briefing rooms remain in use to this day.