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Deep sea mining is the extraction of minerals from the seabed of the deep sea. The main ores of commercial interest are polymetallic nodules, which are found at depths of 4–6 km (2.5–3.7 mi) primarily on the abyssal plain.
But the placer deposits worked in the early years were quickly exhausted, and production crashed. Hardrock mining (in California called quartz mining) began in 1849, and placer mining by hydraulic mining began in 1852. Despite the new mining methods, by 1865 production was 867,000 troy ounces (27,000 kg), less than one-quarter of peak production.