The site began as Spring Quarry, one of the great Bath stone workings that supplied the honey-coloured limestone of Bath and London. In the Second World War it was requisitioned and converted, at enormous expense, into underground aircraft engine factories. Most of the underground manufacturing was abandoned after 1945, but the quarry was already there - dry, deep, immune to anything short of a direct nuclear strike on the chalk above. In the early 1950s, as the Cold War hardened, planners returned. Burlington was developed through the late 1950s and into the 1960s as the Central Government War Headquarters: the place where what was left of British sovereignty would be administered, in absolute isolation, while the country above tried to survive the unsurvivable.

The complex was divided into twenty-two numbered areas, each with a function. Area 14 held the Prime Minister, the War Cabinet, the Cabinet Secretariat, and the Chiefs of Staff. Area 16 contained the BBC studio and the Ministry of Health. Area 18 was the Admiralty, the Army and the Ministry of Defence. Area 19 was workshops and power generation. The kitchens and bakeries were in Area 6; canteen and laundry in Area 12; a hospital in Area 9. An internal Lamson tube system - compressed-air canisters - shuttled messages between areas, because telephone capacity, even with Britain's second-largest exchange installed, was finite. The air could be held at constant humidity and warmed to about twenty degrees Celsius. The lights would have stayed on for three months. The maps and schematics that survive describe an entire civilization compressed into one quarry.

Plans for moving people into the bunker were elaborate and, in their bureaucratic precision, faintly chilling. Four thousand essential workers would assemble at a place called Check Point - the town of Warminster fulfilled that role - from which army lorries would have driven them the final twenty-three miles to Corsham. About two hundred and ten senior Whitehall officials, kept unaware of their destination until the last moment, would have boarded a special train at Kensington Olympia station on the West London line, travelled to Warminster, transferred to buses to the Warminster Infantry Training Centre, and then been broken into small groups for the lorry ride to the quarry. The Prime Minister would have stayed at Downing Street to the very end, then been lifted out by helicopter. The drills assumed the country had hours, perhaps days. Real warning times in the 1980s were thought to be closer to four minutes.

Burlington was never used. The Soviet Union dissolved in 1991, and the Ministry of Defence took over the complex and put it on standby in case of future nuclear threat. In 1989, one section was sold to Octavian Wines, who began using it to store fine wine at constant temperature and humidity - by 2012, around twelve million bottles worth over a billion pounds sat in racks where dormitories were once planned for cabinet ministers. In December 2004 the rest was decommissioned: reservoir drained, fuel removed, staff reduced to four. In 2013, parts of the complex were made scheduled monuments. The most poignant items down there are not the telephone exchanges or the Prime Minister's quarters but the murals - cheerful pre-war paintings by the artist Olga Lehmann, made in 1943 for the underground factory canteen and now Grade II* listed. They were painted to remind workers of normal life. They have outlasted the war they were made for, and the war they were preserved against.

Standing on the surface at Corsham today, there is nothing to see. A few low buildings of MoD Corsham. The undisturbed Wiltshire countryside. The Bath stone walls of a town that has produced limestone for two thousand years. But beneath, in the quiet and the dust, four thousand cots wait. The calculations that went into Burlington imagined the deaths of tens of millions of British civilians, and treated the survival of two hundred civil servants and one Prime Minister as a sufficient continuity of government. That arithmetic is what the place memorialises - a sober reminder of how close, and how routinely, the second half of the twentieth century contemplated its own end.