The other names of adipocere, "grave wax" and "corpse wax," might give you an idea of what it is: This hard, waxy substance is formed in dead bodies under certain special circumstances.
Adipocere is formed only when a body is in a wet, oxygen-free environment without any scavengers, such as insects, to break down the tissues. Under these specific conditions, the body's own fat reacts with water in a process that chemically creates a type of soap! That's right: Soap is made by combining fats with other ingredients, including those present in a waterlogged grave. The result is a waxy, off-white or grayish cast of the body — just don't try to wash with it.