The bricks on the ceiling were layered to form arches so as to disperse the weight from above. These bricks also helped to prevent fire on the aboveground floors from spreading to the basement.
Stains on the walls of the vault are said to be either the result of firefighting, when the fire resulting from the Great Kanto Earthquake of 1923 destroyed the second and third floors of the Main Building, or traces of water used to set the concrete as part of the restoration work following the earthquake.
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