A cube-shaped concrete building was constructed on the top of the hill, next to the dome. This was intended to be used as the bomb-proof outlet for a ventilation and air conditioning shaft. It was an essential component of a facility where dangerous and explosive gases were expected to be used in large quantities on a daily basis. It was never finished, and the Allies found when they captured the site that the ventilation shaft had not been fully excavated. The building survived the bombing intact and is still prominently visible today.