From Wiki. Afsluitdijk (Enclosure Dam), running from Den Oever on Wieringen to the village of Zurich in Friesland. It was to be 32 km long and 90 meters wide, rising to 7.25 meters above sea-level, with an incline of 25% on each side.
This dam runs across the mouth of the Zuiyderzee and in this photo of our Taxi below us, we were standing at the top of the Monument dedicated to the builder Dr Cornelis Lely. Built in the 1920's.
Below from Google Maps today.
Google Map from Amsterdam to the dam over the Zuiyderzee.