Acid rain refers to a mixture of wet and dry deposition (deposited material) from the atmosphere containing higher than normal amounts of nitric and sulfuric acids.  The precursors, or chemical forerunners, of acid rain formation result from both natural sources - volcanoes and decaying vegetation - and man-made sources - primarily emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) resulting from fossil fuel combustion.  When fossil fuels are combusted, acid-forming nitrogen and sulfur oxides are released to the atmosphere.  These compounds are transformed in the atmosphere.  These compounds are transformed in the atmosphere, often traveling thousands of kilometers from their original source, and then fall out on land and water surfaces as acid rain.  Acid rain is best known for the damage it causes to forests and lakes.  Less well known are the many ways it damages freshwater and coastal ecosystems, soils and even ancient historical monuments, or the heavy metals these acids help release into groundwater.