The show route runs through galleries Nazi Germany converted into an underground aircraft-engine works in 1944, walling up passages, blasting rooms wider, and driving air shafts through the cave roof to run machinery worked by about 1,200 labourers, many forced. Retreating German troops set fire to the wooden structures and dynamited the machinery around 12 April 1945 before abandoning the site. The Czechoslovak army then rebuilt the wrecked galleries into a bunker and command post, sealed against fallout with its own power, decontamination gear, and stores to keep a garrison alive underground. That damage compounded an older one: phosphate-clay quarrying from 1920 had already ground most of the cave's Pleistocene bone deposits into fertiliser, and the site was not returned to civilian hands until 2006. Earlier still, in the late 19th century, Vienna Academy of Sciences curator Josef Szombathy opened the cave's shafts and recovered the sediment record, cave bear, woolly rhinoceros, cave lion, hyena, horse, now held at Vienna's Natural History Museum. Long stretches of the original limestone galleries still survive untouched alongside the scars.