This is a Hochbunker in Bremen, with a 1.4 meter thick concrete roof. There was room for approximately 200 people. There were electrical lights and separate toilets/washing facilities for men and women. There were hand cranked air pumping machines to bring in oxygen. The were many very small rooms to prevent people gathering in large groups, to help reduce hysteria.
It has been converted into a music room. Note the additional door clearly showing the thickness of wall required to cut through! The B35 on the business label is probably the official number for this bunker as there is a nearby one know as B31.
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