The cover name was Seewerk, one of a series of camouflaged munitions works, and the construction method was brutally simple: excavate an enormous pit, build inside it, then bury the result under ten to fifteen metres of earth. What went in was a four-level underground plant with a central railway tunnel and two production halls, some 14,000 square metres of usable floor, put up between 1939 and 1943 by the Deutsche Sprengchemie GmbH on a Wehrmacht contract. The Kaiser Wilhelm Institute had been investigating chlorine trifluoride for military use since shortly before the war. Tests were run against mock-ups of the Maginot Line fortifications, and the results were exactly what the army had hoped: a weapon that functioned as incendiary and poison gas at the same time, intended to burn defenders out of concrete they could not leave.

It never worked at scale. By the time the Red Army reached Falkenhagen in 1945, the plant had produced somewhere between thirty and fifty tonnes of N-Stoff in total, at a cost of more than a hundred Reichsmarks a kilogram - an appalling price for a substance so corrosive that containing it was as hard as making it. A sarin production line was planned for the same complex and was never completed. N-Stoff was not used in the war, which is the single piece of good news in the story and is not the same thing as absolution. The intent is on the record in the test results. What the site actually produced, in the end, was an enormous quantity of reinforced concrete, and that turned out to be the valuable part.

The complex was overrun in 1945 and the Soviets, well aware of American nuclear and chemical capability, kept it rather than demolishing it. What research went on there through the 1950s is barely documented; the facility was operational, staffed and closed. Then somebody in the Soviet command looked at fourteen thousand square metres of hardened space under fifteen metres of earth and saw a different use for it. Between 1958 and 1964 the Group of Soviet Forces in Germany rebuilt the interior completely, and in 1965 the Warsaw Pact opened its main command post at Falkenhagen. The chemistry was gone. The concrete stayed.

What the Soviets built inside the old plant was a command centre proofed against nuclear, biological and chemical attack, laced with communications equipment and modernised repeatedly through the 1970s. Western intelligence assessed it as the principal Warsaw Pact bunker outside the borders of the Soviet Union - the place from which a war in Central Europe would have been run. A duty crew was permanently on station. Above ground, the sparse industrial site grew a Soviet garrison village around it: housing, a school, shops, medical facilities, a theatre. Families lived there. Children who grew up in Falkenhagen went to school a few hundred metres above a facility whose purpose their parents were not permitted to discuss, in a country that was not their own.

German reunification in 1990 came with a schedule: Soviet forces would return their bases to German authority by 1994. At Falkenhagen the withdrawal took three years, and most of that time went into removing evidence. Equipment, fittings, documents, signage - everything that identified what the bunker had been for was taken out or destroyed. When the last troops left in 1993 they left hardened concrete and empty rooms. The site passed to the federal property office, was administered until 2003, and was then sold to a Berlin investor. Nothing has been built on it. The garrison village stands derelict in the trees, and beneath it, four levels down, is a plant that two dictatorships found a use for and nobody since has found any use for at all.