A concrete defensive structure used by the German Army in WWII is visible in the sea off the coast of Denmark along the route of the Atlantic Wall (Atlantikwall in German).
The Atlantic Wall (or Atlantikwall in German) was a system of defensive structures built by Nazi Germany between 1942 and 1945, stretching over 1,670 miles (2,690 km) along the coast from the North of Norway to the border between France and Spain at the Pyrenees. The wall was intended to repulse an Allied attack on Nazi-occupied Europe and the largest concentration of structures was along the French coast since an invasion from Great Britain was assumed to be most likely. Slave labour and locals paid a minimum wage were drafted in to supply much of the labour. There are still thousands of ruined structures along the Atlantic coast in all countries where the wall stood except for Germany, where the bunkers were completely dismantled.