But now, new research has uncovered the remains of the mysterious city.
The research was carried out by Kiel University archaeologists, Johannes Gutenberg University Mainz, the Center for Baltic and Scandinavian Archaeology, and the State Archaeology Department Schleswig-Holstein.
The team used geophysical imaging technology and found man-made mounds that had been constructed to protect the town against the tides.
The new findings include the foundations of a church, a harbour and drainage systems.
In a statement, geophysicist Dennis Wilken said: "Settlement remains hidden under the mudflats are first localized and mapped over a wide area using various geophysical methods such as magnetic gradiometry, electromagnetic induction, and seismics."
Dr Hanna Hadler from the Institute of Geography at Mainz University added: "Based on this prospection, we selectively take sediment cores that not only allow us to make statements about spatial and temporal relationships of settlement structures, but also about landscape development."
Dr Ruth Blankenfeldt, an archaeologist at ZBSA also suggested that the "special feature of the find lies in the significance of the church as the centre of a settlement structure, which in its size must be interpreted as a parish with superordinate function."
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