My father was in these pens during several of these attacks when his U Boat was in for service and said that he couldnt even hear or feel anything. He also was in the Pens at Heligoland during raids and said they were vertualy bomb proof. He and my mother went to Heligoland in 57 after the "Britsh Bang" and even detonating that inside the pens did lttle more then blow the doors off.

short but interesting video [www.youtube.com]