The scientific exploration of the ocean is an extraordinary story. Hundreds have climbed Everest yet only two people have descended, in a homespun 'bathyscaphe', to the very depths of the deepest sea chasm. Amazing oases teeming with life have been found in deep sea volcanic vents but we have hardly begun to identify their resident species. We know that sea currents control our climate but we don't know how. Ocean scientists are pretty sure that we could reverse the greenhouse effect by manipulating plankton blooms with doses of iron ... but fear we might trigger an ice age in the act.