What is "mittelsteine"

The Mittelsteine concentration camp was a Nazi Arbeitslager or slave-labour camp functional on the territory of Nazi Germany during the latter part of the Second World War.
It was originally established in 1942, but was operated formally for 250 days (8 months and a week) between 23 August 1944 and 30 April 1945 (the latter being the date of its liquidation) as an all-female subcamp of Gross-Rosen.