"Polymetallic nodules, also called manganese nodules, are rock concretions on the sea bottom formed of concentric layers of iron and manganese hydroxides around a core. Nodules vary in size from tiny particles visible only under a microscope to large pellets more than 20 centimetres (8 in) across. However, most nodules are between 5 and 10 cm in diameter."^^ . "polymetallic nodule"^^ . "2010-03-17T01:14:02Z"^^ . . "ENVO:01000112"^^ . "manganese nodule"^^ . . "pbuttigieg"^^ . "ENVO"^^ .