This report is a contribution to the Strategic Environmental Assessment (SEA4) conducted by the Department of Trade and Industry (now Department of Energy and Climate Change). The report describes the field of mud diapirs in the northern part of the SEA4 area, known as the Pilot Whale Diapirs, which were mapped during a multibeam survey carried out in 2002. A diapir is a fold or dome caused by more plastic rocks at depth, such as salt or mud, intruding overlying denser rocks, generally driven by their own buoyancy. Sometimes a diapir breaks through to the seafloor. The diapir field extends over an area some 60 km across, creating a very variable seabed terrain. The largest diapirs have produced elevations at the seafloor of more than 120m. The largest diapirs in the group are thought to have intruded to the seafloor from 500m or more beneath it. The evidence suggests that the diapirism was initiated between 5 and 1 million years ago, but it does not appear to be continuing at the present time.