manganese nodule

Noun

 * 1)  a concretion of oxides of manganese and iron found on the floor of many oceans
 * 2) * 1977 The most likely locations for early commercial mining of these seabed resources will be the Pacific Basin where there is a heavy concentration of manganese nodule sites at depths of between 12,500 feet and 17,500 feet. The Times - May 26, 1977 pg 27 col D