That took an incredible amount of labor. It’s something I would expect Kim Jong Un of North Korea to do; endless, seemingly pointless hard labor.
What was the incentive for the other Germans to do it?
Were they prisoners? No OSHA in those days. No Labor Unions, No counseling for PTSD either, claustrophobics were on their own.
Zhey weh nacht prissonahs... zhey koult liv any time. But!, wheh voult zhey goh?