The team of scientists from Japan's Agency for Marine-Earth Science and the University of Tokyo first discovered huge reserves in the mid-Pacific two years ago.
These are now thought to be 1000 times all land-based deposits, some of it in French waters around Tahiti.
The latest discovery is in Japan's Exclusive Economic Zone in deep-sea mud around the island of Minami-Torishima at 5,700 meters below sea level. Although it is very deep, the deposits are in highly-concentrated nodules that can be extracted using pressurised air with minimal disturbance off the seafloor and no need for the leaching.
Professor Kato said exploration will continue for another two years before scaling up towards production.
Over 50pc of the metal in the deposit is the heavier end of the spectrum, twice the level of China's key mines and without the radioactive by-product thorium that makes the metals so hard to mine
Professor Kato said a single ship drilling in the target zone at Minami-Torishima could supply Japan's needs for a year, breaking strategic dependence at minimal cost. "We don't need to mine it intensively. All we need is enough to force China to lower its prices."
The new discovery is the second time this month that Japan has announced a major find on the sea floor.
It announced a break-through in extracting gas from methane hydrates under the ocean last week, a technology that is likely to prove costly but could meet Japan's gas needs for a century.
Japan breaks China's stranglehold on rare metals with sea-mud bonanza - Telegraph
