Marsdiep
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The Marsdiep is a deep tide-race between Den Helder and Texel in the Netherlands, and running southwards between sandbanks. That gap connects the North Sea and the Waddenzee.
Around 1000 AD and before, much of the modern Waddenzee and IJsselmeer was land and the Marsdiep was an ordinary river. An early form of its name is Maresdeop, which likeliest means "moeras diep /swamp deep". For more information see Wieringen.
The Marsdiep was in the age of sail in English called the Texel, in distinction from the island of Texel.