Erected between 1941 and 1942 at a cost of 400,000 Reichsmark, roughly 1.69 million euros in today's money, the Schwerbelastungskörper, the 'heavy load-bearing body', is laboratory equipment at architectural scale. The visible cylinder rests on a foundation eleven metres across that drives 18.2 metres into the earth, and within that foundation are rooms that once held the instruments measuring how far the mass settled. The load was calculated to match what a single pillar of Speer's planned arch would exert on the ground. The arch was to stand about three times the size of the Arc de Triomphe in Paris, anchoring the southern end of a boulevard cut through the middle of the city. Nothing of that weight had ever been attempted on ground like this, and Berlin's ground is not rock but sand, gravel and old marsh.

The arch was one component of the capital Hitler and Speer intended to build and call Germania. Its centrepiece was the Volkshalle, a domed assembly hall 290 metres high with space for more than 180,000 people. When Speer described the design to British and American structural engineers in 1945, they told him the interior would have been so vast that condensation from the breath of a full crowd would have formed clouds inside it.

None of it was to rise on empty ground. Rebuilding the centre of Berlin meant demolishing thousands of homes, and Speer's General Building Inspectorate solved the resulting housing shortage by taking housing from Jewish Berliners: from 1939 his office used the Nuremberg Laws to clear some 15,000 flats of their Jewish tenants, who were crowded into designated houses and, before long, put on deportation trains. The same office built and administered forced-labour camps across the city. The monumental capital was to be assembled out of other people's homes and other people's labour, and the accounting for that began years before the first stone would have been laid.

Engineers read the instruments for two and a half years. A settlement of less than six centimetres would have shown the ground was suitable. The cylinder sank nineteen. Measurements stopped in June 1944, by which point the finding was academic: the arch could have been built only after extensive stabilisation of the subsoil, and there was no longer a state capable of stabilising anything. The most honest verdict on Germania came not from an architect or a historian but from the sand under Tempelhof, which simply gave way and kept giving way.

After the war the obvious course was demolition, and nobody could work out how. A charge large enough to break 12,650 tonnes of concrete would have taken the neighbouring rail tracks and apartment buildings with it. So the cylinder stayed where it was, the city grew back around it, and for decades Berliners walked past without knowing what they were passing. In 1995 it was placed under monument protection, described as the only tangible relic of National Socialist urban planning left in the city. It is open to the public, with guided tours and panels beside it setting out what Speer intended to put here.

There is something clarifying about the object. Berlin holds no shortage of Germania drawings, models and newsreel, all of it seductive in the way unbuilt architecture always is; the scheme survives in the imagination precisely because it never had to meet a wall, a budget or a water table. This is the one part poured at full size, and it is not a gate, a hall or a monument. It is a weight. Stand beside it and you are looking at the exact quantity of concrete one column of the arch would have driven into the earth, and at the answer the earth returned.