Field observations and laboratory testing were used to conclude that aggressive localized corrosion of carbon steel pilings in Duluth-Superior Harbor, Minnesota and Wisconsin, is caused by the following sequence of biological, chemical, and physical events. Iron-oxidizing bacteria colonize the carbon steel sheet pilings and produce tubercles, which are made up of intact and/or partially degraded remains of bacterial cells mixed with amorphous hydrous ferric oxides. The reducing conditions beneath the tubercles cause copper dissolved in the water to precipitate. A galvanic couple is established between the copper layer and the iron substratum. Ice scouring breaks the tubercles. Exposure of the copper-covered iron to oxygen causes the galvanic current to increase. The result is aggressive localized corrosion.