Towards the end of WWII the Seegrotte was requisitioned by Heinkel as an ideal 'bomb-proof' location for the production of the the He 162 jet with upwards of 2,000 workers/slave labourers being employed in the 'factory'. Many came from the infamous Mauthausen concentration camp, one of the first to be constructed from stone outside of Germany and one of the last to be liberated. The oft-reproduced picture above is published in the work ' Mauthausen, Horror on the Danube'. After the war the Seegrotte was re-opened as a tourist attraction. The following views are/were available as postcards..
Me 262 construction at the REIMAHG underground facility
Me 262 'forest-factory' production