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.
Huge rare earth deposits found in Pacific: Japan experts Reuters