"The deposits have a heavy concentration of rare earths. Just one square kilometer (0.4 square mile) of deposits will be able to provide one-fifth of the current global annual consumption," said Yasuhiro Kato, an associate professor of earth science at the University of Tokyo.
The discovery was made by a team led by Kato and including researchers from the Japan Agency for Marine-Earth Science and Technology.
They found the minerals in sea mud extracted from depths of 3,500 to 6,000 meters (11,500-20,000 ft) below the ocean surface at 78 locations. One-third of the sites yielded rich contents of rare earths and the metal yttrium, Kato said in a telephone interview.
The deposits are in international waters in an area stretching east and west of Hawaii, as well as east of Tahiti in French Polynesia, he said.
He estimated rare earths contained in the deposits amounted to 80 to 100 billion metric tons, compared to global reserves currently confirmed by the U.S. Geological Survey of just 110 million tonnes that have been found mainly in China, Russia and other former Soviet countries, and the United States.
Details of the discovery were published on Monday in the online version of British journal Nature Geoscience.
The level of uranium and thorium -- radioactive ingredients that are usually contained in such deposits that can pose environmental hazards -- was found to be one-fifth of those in deposits on land, Kato said.
A chronic shortage of rare earths, vital for making a range of high-technology electronics, magnets and batteries, has encouraged mining projects for them in recent years.
China, which accounts for 97 percent of global rare earth supplies, has been tightening trade in the strategic metals, sparking an explosion in prices.
Japan, which accounts for a third of global demand, has been stung badly, and has been looking to diversify its supply sources, particularly of heavy rare earths such as dysprosium used in magnets.
Kato said the sea mud was especially rich in heavier rare earths such as gadolinium, lutetium, terbium and dysprosium.
"These are used to manufacture flat-screen TVs, LED (light-emitting diode) valves, and hybrid cars," he said.
Extracting the deposits requires pumping up material from the ocean floor. "Sea mud can be brought up to ships and we can extract rare earths right there using simple acid leaching," he said.
"Using diluted acid, the process is fast, and within a few hours we can extract 80-90 percent of rare earths from the mud."
The team found that sites close to Hawaii and Tahiti were especially rich in rare earths, he said.
He gave no estimate of when extraction of the materials from the seabed might start.
(Reporting by El Tan in Hong Kong and Yuko Inoue in Tokyo; Editing by Michael Watson)