The physical difference between a liquid and a highly compressed, very dense gas is subtle. Unlike the well-defined surface between Earth’s atmosphere and its oceans, on Jupiter and Saturn there is no clear boundary between the atmosphere and the ocean of liquid hydrogen and helium that lies below. The depths of these hydrogenhelium oceans are measured in tens of thousands of kilometers, making them the largest structures within the interiors of any of the giant planets. Uranus and Neptune are less massive than Jupiter and Saturn, have lower interior pressures, and contain a smaller fraction of hydrogen. Their interiors probably contain only a small amount of liquid hydrogen, with little or none of it in a metallic state, and they do not have these oceans of liquid hydrogen and helium.