In 2011 a systematic underwater field survey and an invasive investigation were executed in the Yangtze Harbour planning area, Maasvlakte, Rotterdam, commissioned by Port of Rotterdam Authority. The aim of the work was to locate and document any archaeological remains in submerged Late Pleistocene and Early Holocene deposits at a depth of 22m to 17m bsl, in the to-be-deepened and extended harbour. A drowned Lateglacial-Early Holocene floodplain landscape was mapped and a Mesolithic site was discovered in it. The site was visited from c. 8500 to 6500 BC and became a nodal base camp 7000 to 6500 BC, when a wetland landscape had developed around it. Abundant flint tools, bones (of mammals, fish and bird) and plant material (pristine and charred) were recovered and provide information on mobility, diet and use of fire in the Middle Mesolithic. The site rapidly drowned in the transgressing North Sea owing to accelerated sea-level rise events of the period 6500-6300 BC. The site offers a window to the lowland archaeology of the Early Holocene in areas now offshore, that inland sites cannot off. The discoveries allow interregional comparison of delta habitation between the Middle Mesolithic (offshore, at depth in the sea, Doggerland) and Late Mesolithic (onshore, Rhine-Meuse delta, at depth below urbanized coastal plain).