Edam Fort in Edam-Volendam

You're standing at the Fort near Edam, in the Zeevang polder, just north of the Edam lock. Look at the big grassy mound. Hidden underneath is the fort, a concrete building covered with sand. It's part of the Defence Line of Amsterdam, a ring of fortifications built around the capital between roughly 1880 and 1920. Today that whole ring is a World Heritage site.