3D model of a Dutch bunker type S3 'stekelvarken' ('porcupine'). This bunker type was designed in 1939 and housed one light machine gun. The gun could be positioned in one of the three hatches, and the other two could be locked off with a 2cm thick steel door. The bunker's walls were made of 80cm thick reinforced concrete. A total of 763 bunkers of this type have been built in the days before WWII in the Netherlands. During the invasion by the German army in may 1940 the S3 bunkers proved to be vulnerable because of their relatively thin walls and big shooting holes. Source: http://www.grebbelinie.nl/page/s3