Acid Mine Drainage (AMD, sometimes also called Acid Rock Drainage) is a result of the oxidation of minerals containing chemically reduced forms of sulfur, upon exposure to water and air. For example, pyrite is a metal sulfide (i.e., the sulfur is an anion with a -1 charge, and the iron is a cation with a +2) that can be oxidized in the presence of air and water to produce sulfuric acid and ferric iron hydroxide:

2FeS_{2 (s)} + 15/2 O₂ + 7 H₂O --> (8H⁺ + 4SO₄^2-) + 2Fe(OH)_{3 (s)}

Around mines, this chemical phenomenon can happen as material that contains the metal sulfides is broken up into small pieces and a lot of surface area is exposed. If water washes over that material, the acid can be generated and flow into nearby waterways. AMD is commonly observed in seeps from underground mines also (i.e., wherever water is flowing out of the mine). It is frequently associated with coal mines (since pyrite is often found at the interface between the coal seam and the overburden/interburden) and metal mines where the ore deposit contains sulfide minerals.

Once the acidic mine water flows out, it drains into streams, rivers, lakes, etc. and can cause harm to the environment/ecosystem of that area. If the acid concentration is high enough, it can cause harm to any organisms in the body of water or nearby the body of water that is affected. A visible indicator of acid mine drainage is called "yellow boy." This is an orange-colored substance that forms as the dissolved iron hydroxide precipitates out of solution. Impacted streams can have a pH as low as 4 which corresponds to the pH of battery acid.

Figure: Acid mine drainage (AMD) is a prominent environmental concern in Appalachian coal mining.