Extensive preparatory work had to be done before the excavation work could begin for the underground levels of the Potsdamer Platz area – up to four floors of them.

There had been no complete "clearing out", that is, removing all traces of the ruins and remains of ammunition left in the ground after the Second World War, at Potsdamer Platz before – so that had to be done now. This resulted in many interesting artifacts being revealed, including porcelain cups from "Café Josty", the famous literature cafe at the old Potsdamer Platz. But there were lots of pieces of ammunition and dud bombs from the war that had to separated by hand from the so-called "cultural rubble" and rendered safe.

The foundations situation is very difficult at Potsdamer Platz. To a great depth, the ground consists of sand, clay and loose stones. The groundwater level is only about three meters deep. Although sand is easier to remove than rock, the groundwater soon sets the entire material in motion. So sand and water had to be held back during the entire foundation process while the lagging and sheeting were being out into place.

There were lakes in the middle of Berlin for months. The picture of Potsdamer Platzes was dominated by boats and pontoons. Like at sea, life jackets had to be worn; life belts were installed along the excavations. And divers were at work under water: quite a sensation in the middle of Berlin. The divers worked at depths of up to 23 meters. They prepared the bottom of the construction lake for concrete to be laid. In the cold winter of 1995/96, 80 divers were at work around the clock. With ambient temperatures averaging -15°C, the surface of the water continually had to be kept free of ice. Deep under the water, the temperature was a constant +2°C. "Divers at Potsdamer Platz" was one of the most popular subjects for reporting about the new capital city – the construction site became a postcard picture.