1. ENVIRONMENTAL FACTORS. Since 1975 the inadequacy of knowledge of the depth and shape of the seabed on the continental shelves has become increasingly apparent, as navigational safety has been pushed to further limits in response to economic factors. Less than half of the British continental shelf, defined approximately by the 200 m depth contour, has been surveyed by echo-sounder and only a small proportion to full modern standards. There are inevitably many natural hazards still to be discovered. Inadequate knowledge can have particularly serious effects in heavily trafficked shallow areas, where patches of the seabed may show instability in the form of mobile sandwaves. Seabed wrecks and obstructions, some of them wartime relics, are another problem; thanks to the use of modern sidescan sonar, in 10 years the number of wrecks in the UK area databank has risen from 14000 to 22000.