As early as 1934, urban planners and architects of the Third Reich are building representative large-scale constructions, whose basements exceed all previous buildings in size. The expansion of the Reichsbank is a typical example with three underground levels and the largest “vault” in Germany. Tempelhof Airport, at that time the biggest building in the world, also has more than 4.3 kilometres of walk-in supply ducts in addition to an underground railway connection, in which the entire infrastructure required for such a large-scale construction is located. In order to prepare the transport system in Berlin for the Olympic Games in 1936, construction of the north-south commuter train line tunnel begins in 1934. The new ruling power can fall back on sophisticated plans from the 20s for these large-scale projects. In addition, the implementation of these plans also makes use of extremely effective propaganda as, in one go, thousands of workers are employed, who more or less excavate the tunnel by hand. Over the construction site hang slogans like: “That we build here, we owe to the Führer”. Construction is overshadowed by the worst disaster in Berlin's civil engineering history.
South of the Brandenburg Gate, directly in front of the old American Embassy, on August 20th, 1935, 50 metres of the over 14-metre-deep tunnel excavation collapses. Dozens of construction workers are buried by falling casings, coverings and wooden scaffolding. 19 of them die. The cause of the tragedy is later determined to be a lack of stability in the excavation pit supports. However, the enormous time pressure because of the upcoming Olympics, as well as rescheduling the tunnel at short notice for the transformation of Berlin should be seen as other causes. Even the elaborate funeral service is under time pressure. The recovery of the dead drags on and only 17 of the 19 casualties can be laid out. Although the families of the victims receive compensation, the promise to immortalise the names of the victims in "metal lettering" at Potsdamer Platz station is never honoured. Nevertheless, these efforts are too late for the Olympic Games. Potsdamer Platz station is only inaugurated on April 15th, 1939, the north-south tunnel becomes fully operational just under six months later.
In 1938, under the aegis of Hitler’s chief architect, Albert Speer, reconstruction measures begin for the Capital of the Reich, “Germania”. At the heart of these plans is the so-called north-south axis, in the centre of which the “Great Hall” will be built. Over 300 metres high and with space for 180,000 people, it is to become the largest building in the world. Around four kilometres south, Hitler’s triumphal arch is planned, with nine times the volume of the Arc de Triomphe in Paris. Albert’s Speer’s father, who views the “Germania” model in 1939, has only this to say to say to his son about the “megalomaniacal world power architecture”: “You’ve gone completely mad”. In 1940, most of the construction projects are interrupted for the time being and, at the end of 1941, mostly postponed as "not important to the war effort", but detailed plans are drawn up until 1943. Eventually, only a few small parts of “Germania” are realised, including the tunnel system for the so-called axis cross as well as the “House of Tourism” near Potsdamer Platz, whose ruins were only removed in 1962.
Long before the outbreak of war, underground air-raid shelters and bunkers play an important role in the strategic plans of the National Socialists. From 1935, air-raid shelters must be constructed under all new buildings. In the beginning, the ceilings are still quite thin. After the German invasion of Poland on September 1st, 1939, the trigger for the Second World War, the first heavy bombing attacks on Berlin soon follow. In November 1940, the “Bunker Construction Programme for the Capital of the Reich” is announced. Within its framework, around 1,000 bunkers and air-raid tunnels are to be built. Thus, all ministries and embassies are equipped with at least one bunker. However, Berlin’s bunkers are only sufficient for just under 25% of the population during the Second World War, despite enormous efforts and three to four times overcrowding. During the bombing, most city dwellers are left with insufficient protection in trench shelters, makeshift basements, or in so-called "public air-raid shelters" set up at various locations below the city. Large areas of the city centre and the urban infrastructure are eventually destroyed, and millions of war dead are mourned as Adolf Hitler commits suicide in the best-known bunker in the city, the so-called Führerbunker, on April 30th, 1945.
As late as March 9th, 1945, the general order for the defence of Berlin states that the battle for Berlin must be conducted using all means, "the subway and the underground sewer network above and below ground." However, the insufficient preparations for the so-called “final battle” are less than useless. Rubble debris is piled up at the most important intersections to form “anti-tank barriers”, and the gaps are to be closed at the last minute with destroyed trams or furniture vans. In the subway tunnels, temporary barricades of bags of sand and wooden boxes are erected. The underground commuter train and subway stations turn into a huge army camp, crammed with civilians and wounded, next to the support bases of the last defenders. Many bunker systems are also converted into command posts, ammunition and food are stored. From April 30th, 1945, a tragedy takes place in the commuter train tunnel. The SS have the high-rise bunker at Anhalter Bahnhof evacuated as they enter the war zone. Thousands of old people, women and children are driven through the tunnel connected to the bunker in the direction of Friedrichstrasse and Stettiner station. The transport tunnels remain largely in the hands of the defenders until the end of the war, who bring in underground reinforcements and munitions. It’s even more extraordinary that in the early hours of May 2nd, 1945 – after almost all of the hostilities had ended – the commuter train tunnel under the Landwehr Canal is blown up by the SS. The water floods the whole underground transport network in the city centre. Fortunately, hardly any civilians are in the tunnels, only a few people drown in the floods. Whether intentional, meaningless vandalism based on the principle of "scorched earth" or just "a stupid coincidence" - the reason for the demolition will probably always remain a mystery of history.