While it is a commonly-known fact that oil (petroleum specifically) and water do not mix, this is not true when one or both are changed through artificial means. Normally, oil remains deep underground while water is present in the atmosphere and in liquid form on the surface of the Earth. Harvest and use of oil causes the oil molecules to break down, but not completely; oil exhaust (sulfur dioxide) combines with nitrous oxide and water vapor in the atmosphere to create Acid Rain, which is harmful to man-made structures and to the environment at large. Acid rain can cause severe damage to fragile ecosystems, and harm both plants and animals. Another issue is that of atmospheric pollution; incompletely degraded oil molecules release carbon monoxide and other harmful gasses, some of which can harm animals and humans who breathe it, and some of which can become greenhouse gasses, potentially increasing climate temperature. Additionally, burning oil releases a certain amount of volatile organic compounds, which through their interaction with ultraviolet light help to deplete the ozone layer (interestingly, at ground level, ozone itself is considered a pollutant). In this way, water and oil are both involved in damaging the environment and atmosphere, although most of this damage is directly related to human activities.