Polymetallic manganese nodules:
They are in the sizes ranging from a potato to a dining table and are available in plenty on sea floors at the depth of 4,000 metres to 6,500 metres. They have been known since the 1860s and were first mentioned in HMS Challenger expedition. They contain nickel, copper, lithium besides other minerals. The Clarion Clipperton Zone of the Pacific has significant deposits: 15 kg per sq m. This entire zone contains 62 billion tonnes of such nodules.