It's the former Nazi camp called Mittelbau-Dora. It was a forced labour camp rather than a death camp. However, the numbers of inmates that died was awful.
It's on the outskirts of a town called Nordhausen. How they didn't know it was there is hard to believe. I have been before and I feel once in the area it would be good to drop in and take a look again.
More info on Wiki: https://en.m.wikipedia.org/wiki/Mittelbau-Dora_concentration_camp
