Manganese nodules occur as two-dimensional deposits in abyssal plains of all major oceans. In the Clarion-Clipperton Zone of the northeast equatorial Pacific alone, the amount of nodules is estimated to 21 billion tons indicating the huge potential of this deposit type. Apart from manganese, metals of economic interest are nickel, copper, and cobalt, but the nodules also contain interesting amounts of molybdenum, titanium, lithium, and the rare earth elements. Therefore they are also called as polymetallic nodules.