Coal, when burnt, gives off almost a third more carbon dioxide per unit of energy than oil, and 80% more than natural gas. Almost half of the carbon dioxide from people is because of burning coal so it is the biggest single cause of global warming. Coal contributes to acid rain and smog, especially when burned without scrubbers. Burning coal releases toxic chemicals, including soot, mercury, and carbon monoxide, which contribute to diseases such as cancer and asthma in both humans and wild animals. Coal mining, especially mountaintop removal mining, can damage large areas of land and destroy natural habitats. Higher grades of coal burn more cleanly than lower grades, although they still pollute more than other fuels. In addition to air pollution, burning coal produces toxic coal ash, which can cause water pollution if it is accidentally released into the environment. There are several underground mine fires burning throughout the world. These underground fires release toxic smoke into the air, and can also cause the ground above to collapse. The city of Centralia, Pennsylvania was evacuated and is now abandoned due to an underground coal mine fire.