'Composed of over sixty per cent water itself, a human body isn't naturally buoyant. It will float only for as long as there is air in its lungs, before gradually sinking to the bottom as the air seeps out. If the water is very cold or deep, it will remain there, undergoing a slow, dark dissolution that can take years. But if the water is warm enough for bacteria to feed and multiply, then it will continue to decompose. Gases will build up in the intestines, increasing the body's buoyancy until it floats again. And the dead will literally rise . .