The three Dunkirk transgressions are events of marine transgression (rising seas) around the shores of the Low Countries in the late Roman period.
Soil survey geological evidences and lack of human occupation artefacts led scientists to formulate the claim that the Netherlands was largely underwater between the 3rd century and 1050. This is the so-called third Dunkirk Transgression.
The reality of the events, hypothetically determined by cyclical phases of strong sea level rises in historical times, is debated.
The low-lying continental coast of Europe was lightly populated until c. 200 BC, when the climate and environment became more amenable to human habitation. Conditions remained favourable from 200 BC to 250 AD, and the region became densely populated.
However the region had been undergoing a series of marine transgressions (called Dunkirk 0 through Dunkirk IIIb) characterised by a rising water table and floods that left layers of clay on the land. The heaviest blow came with the "Dunkirk II transgression" that began in the 3rd century and continually worsened, leaving large areas of the coast uninhabitable from c. 350–c. 700. People were forced to abandon their homes and emigrate. Archaeologists conducting research along the historically flood-prone coast tell this same story for The Rhine/Meuse delta (Zeeland, Brabant, parts of South Holland and Limburg); Friesland; Groningen; Ostfriesland, German Friesland and the Weser/Jade estuary; and Dithmarschen, Eiderstedt and Nordfriesland.
In the Rhine/Meuse delta, the population became scanty. Between the 5th and 7th centuries there were few centers of occupation in the delta region, and in the estuarine and peat areas no settlements at all have been found. The area would not be repopulated until the Carolingian Era. The areas with river clay were so covered with sedimentation that habitation was almost impossible between 250–650.