One of the most difficult challenges faced by the Germans was constructing the great dome while under regular air attack. The dome's designer, Todt Organisation engineer Werner Flos, devised a plan under which the dome would be built first, flat upon the ground, and the soil underneath it would be excavated so that the construction works below would be protected against aerial attacks. A circular trench was excavated on the top of the hill above the quarry to an outside diameter of . The dome was built within this trench and the galleries and octagonal preparation chamber were excavated below.