Shortly before the liberation, in the summer of 1944, a 5,000 square metre workshop was set up in the Heidegroeve for the production of parts for aircraft radios. Almost a thousand (forced) labourers would work there day and night. At least, that was the intention. But the rapid Allied advance in August and September threw a spanner in the works.

Because of the increasing number and scale of Allied bombardments, part of the German war production went underground from 1943 onwards. This switch was tackled 'gründlich' (thoroughly), also in Valkenburg. From March 1944 onwards, nearly three hundred forced labourers worked continuously in the cave to build the workshop. A generator provided electricity, and gas and water pipes were laid from the town. The Heidegroeve was fitted with an air conditioning system, including boilers, to regulate humidity and temperature. A large bunker was built on top of the Cauberg in which the enormous boilers were placed.

A 46-metre shaft led through the transformer room near the workshop to the boiler room. The stepped shaft also served as an emergency exit in case the main entrance on Plenkertstraat became blocked. A new ventilation system ensured the supply and removal of air. Bunkers were built on the existing ventilation shafts to prevent enemy projectiles from entering the quarry.

On 23 June 1988, a collapse occurred at the Heidegroeve, with more than half of the quarry going under. The 46-metre-long shaft and the engine room were also lost.