WW2 U-boat pens or bunkers used by the Germans to protect their submarines from air attack.
https://diqn32j8nouaz.cloudfront.net/ww2_uboat_pen.jpg
https://diqn32j8nouaz.cloudfront.net/ww2_uboat_pen1.jpg
https://diqn32j8nouaz.cloudfront.net/ww2_uboat_pen2.jpg
https://diqn32j8nouaz.cloudfront.net/ww2_uboat_pen3.jpg
Previously:
https://www.homemadetools.net/forum/...204#post130538
https://www.homemadetools.net/forum/...357#post154472
https://www.homemadetools.net/forum/...228#post153978
https://www.homemadetools.net/forum/...098#post153555
https://www.homemadetools.net/forum/...4385#post79850