The idea is to store the carbon dioxide produced by the burning of fossil fuels under the sea, stopping it from getting into the atmosphere and adding to the greenhouse effect. The high pressures and low temperatures on the seabed should force liquid carbon dioxide to form a solid known as gas hydrate - an icy mixture of water and carbon dioxide.
Gas hydrates occur naturally in many parts of the world.
Tests carried out in the laboratory showed enough promise for researchers from the Monterey Bay Aquarium Research Institute in the United States to carry out an experiment 3.5km below the Pacific, using a remote-controlled submarine. This took flasks of carbon dioxide down to the sea floor, let it leak out and then watched what happened using a video camera. The gas did react with the seawater to form hydrates, but these expanded rapidly and started to dissolve. At shallower depths of around one kilometre, these rose up towards the surface, meaning they would quickly return to the surface. At 3.5km the gas hydrate drifted off and dissolved, but did not sit stably on the sea floor.