Methane is a significant and plentiful fuel which is the principal component of natural gas. Burning one molecule of methane in the presence of oxygen releases one molecule of CO2 (carbon dioxide) and two molecules of H2O (water). Methane's relative abundance and clean burning process makes it a very attractive fuel. However, because it is a gas and not a liquid or solid, methane is difficult to transport from the areas that produce it to the areas that consume it. Converting methane to forms that are more easily transported, such as LNG (liquefied natural gas) and methanol, is an active area of research. Methane is a greenhouse gas with a global warming potential over 100 years of 23. This means that when averaged over 100 years each kg of CH4 warms the Earth 23 times as much as the same mass of CO2.The Earth's crust contains huge amounts of methane. Large amounts of methane are emitted to the atmosphere through mud volcanoes which are connected with deep geological faults or as the main constituent of biogas formed naturally by anaerobic digestion.