Den Helder, the Helderse Zeedijk, is originally a weaker dike that was built in 1584, after the dune area disappeared it became a sea dike that was constantly strengthened over the years, so the construction of the closing dike led to the installation of a retaining wall which disappeared under the dike after it was raised to Delta height in the 70s..© 30 juni 2018 Marco van Middelkoop/Aerophoto-Schiphol