About three hours after death, rigor mortis sets in. Around 12 hours after death, the body will feel cool, and within 24 hours (depending on body fat and external temperatures), it will lose all internal heat in a process called algor mortis. The muscle tissue begins to lose its stiffness after about 36 hours, and within about 72 hours of dying, the body's rigor mortis will subside. As the cells die, bacteria within the body begin breaking them down. Enzymes in the pancreas cause the organ to digest itself. The body soon takes on a gruesome appearance and smell. Decomposing tissue emits a green substance, as well as gasses such as methane and hydrogen sulfide. The lungs expel fluid through the mouth and nose. The environment in which a dead body is placed also affects its rate of decay. For instance, bodies in water decompose twice as fast as those left unburied on land. Decomposition is slowest underground -- especially in clay or other solid substances that prevent air from reaching the body since most bacteria require oxygen to survive.