Carrier transported liquid CO2 at -55degreesC is denser than ambient seawater at mid-ocean depths. We have investigated whether this property effectively enables sinking of injected CO2 from mid-depth to the ocean floor, > 3500 in depth, where CO2 is gravitationally stable as a lake on the dented sea floor. In order to obtain basic data for the realization of this idea, the National Maritime Research Institute, and the Monterey Bay Aquarium Research Institute, conducted three joint in situ experiments of CO2 sending method for the ocean storage (COSMOS), to release cold CO2 at the mid-ocean depths. The experiments were carried out in Monterey Bay from October 1999 to February 2002 using remotely operated vehicle (ROV) techniques to effect the controlled release and subsequent imaging. From the data obtained, it was clear that a cold CO2 mass, released as a large unit, was apt to be broken up into small droplets by a Taylor type interface instability. Even for a unit of sufficient heat capacity for formation of a significant ice layer, break up into droplets due to liquid instabilities occurred in a short time. However, in experiments with a CO2 slurry mass (a mixture of dry ice and liquid CO2) of 8 cm size we observed that the released material could keep its shape and sink even further until the covering ice layer melted. The behavior of the CO2 slurry mass strongly suggests that this technique offers the potential for effective transfer of released CO2 from mid-depth to the ocean floor, and our experiments provide numerical constraints on the required design goals for this. (C) 2004 Elsevier Ltd. All rights reserved.