From the end of 1943, concentration camp prisoners had to dig tunnels for underground arms factories that were protected from bombs.
In Gusen, Ebensee, Melk and Peggau and other subcamps, prisoners were deployed in tunnel construction. The largest tunnel system was built in St. Georgen an der Gusen under the codename ‘Bergkristall’. Many thousands of people lost their lives as a result of this gruelling forced labour.