The North Sea and the neighbouring land areas of Denmark, Germany, the Netherlands and the UK make up one geological unit, a sedimentary basin with clearly defined structural borders.1 To the north-west of the basin is the Caledonian folding, which makes the mountains of Ireland, Scotland and Norway. The north-eastern limit is the Baltic-Scandinavian shield, and the southern edge of the basin is the Hercynian range. To the north the region is bounded by the deep-sea basin of the Atlantic ocean.