Japanese researchers have discovered significant amounts of rare earth deposits on the bottom of the ocean about 1,243 miles southeast of Tokyo.

"An estimated 6.8 million tons of rare earth minerals, including dysprosium, exist in the mud in the deposit, which is within Japan's exclusive economic zone," said the group headed by University of Tokyo Professor Yasuhiro Kato, an expert in earth resources, Japan's state-run news agency Kyodo reports.

The amount is enough to supply Japan with the minerals for 230 years, the research team says.

Japan now relies on its supplies of rare earths from China, which has a monopoly of the 17 minerals used in the electronics, defense and renewable energy industries.

While China's rare earths reserves represent one-third of the global totals, it supplies more than 90 percent of the world's supply of the minerals. Since 2010, China has further tightened its grip on rare earths by introducing a series of export quotas on the minerals, arguing the measures are necessary to protect the country's natural resources and environment.

Kato told the Wall Street Journal Tokyo-based Mitsui Ocean Development & Engineering Co., a company that specializes in deep-sea oil exploration, will work with his team in surveying the area. He estimates underwater mining for the rare earths could start in three to five years.

Industry insiders, however, say development of resource-rich areas in the Pacific Ocean could take as long as 20 years, the Journal report says. But that timetable factors in any likely international territory disputes in the areas and wouldn't apply to the under-sea rare earths discovery because it is located in Japan's exclusive economic zone.

Kato said he expects the cost to develop the underwater minerals will be lower compared with land-based rare earths because sea mining will not require the disposal of traces of radiogenic elements as does land mining.

"When people hear about developing resources they tend to immediately assume it will come with high costs. But, in fact, the cost of developing it is comparable to how much it costs to import rare earths now," Kato told the Journal.

While the Japanese government welcomed news of the discovery, it indicated it wasn't ready to rush into development.

"This is welcome news," said Japan's Minister of Economy, Trade and Industry Yukio Edano. "But the professor also says that a more detailed survey is necessary and that technology has to be developed to make commercial development possible," Edano said, adding the ministry had been planning to conduct its own survey of the area and would cooperate with the research team.

Last week, Japan, together with the United States and the European Union, requested that the World Trade Organization establish a panel to determine whether China's export restrictions on rare earths are permissible under WTO trade rules.