The United States' biggest export, in tons and dollar value, is eroding topsoil.

North American topsoil pours into the ocean at the mouth of the Mississippi River, here seen from space. As topsoil disappears, farmers need more fertilizer to keep crop yields high. Fertilizer runoff is a major cause of the oxygen-depleted "dead zone" in the Gulf of Mexico, which claimed 22,000 km² (8,500 mi²) in 2002 -- an area the size of Massachusetts.