Scientists believe areas of enhanced electrical conductivity in the mantle - the thick region between the Earth's crust and its core - betray the presence of water. Water divining researchers produced a global three-dimensional map of the mantle showing the areas through which electricity flowed most freely. Conductivity hot spots were found to coincide with subduction zones, sites where the tectonic plates that divide up the Earth's surface are being forced downwards. This was a surprise since subducting plates are colder than the surrounding mantle rock, and should therefore be less conductive. The anomaly is best explained by water being drawn downwards by the subduction process and increasing conductivity, say the scientists. ''The model clearly shows a close association between subduction zones and high conductivity, and the simplest explanation is water,'' said Professor Adam Schultz, from Oregon State University in the US, who co-led the study published in the journal Nature. Colleague Professor Gary Egbert, also at Oregon State, added: ''In fact, we don't really know how much water there is on Earth. There is some evidence that there is many times more water below the ocean floor than there is in all the oceans of the world combined. Our results may shed some light on this question.'' Other possible reasons for enhanced conductivity in the mantle include raised levels of iron or carbon, said the researchers. The presence of water could also be explained in different ways. ''If it isn't being subducted down with the plates, how did it get there?'' said Prof Schultz. ''Is it primordial, down there for four billion years? Or did it indeed come down as the plates slowly subduct, suggesting that the planet may have been much wetter a long time ago? These are fascinating questions, for which we do not yet have answers.'' The scientists used three decades of magnetic field soundings made by more than 100 Earth observatories to construct their map. Next they hope to repeat the study with more recent data both from ground bases and satellites.