Researchers from the Japan Agency for Marine-Earth Science and Technology, or JAMSTEC, conducted a robot survey of the deposit about 350 kilometers off Chiba Prefecture from April to May.

The area with cobalt-rich crusts was found nearly 30 years ago. It contains a large amount of cobalt and other rare metals.

The researchers say the surface of a submarine volcano at the depth of 1,500 to 5,500 meters was covered with the crusts.

The crusts extend for about 950 square kilometers and are more than 10 centimeters-thick. That's about 2 times as thick of cobalt-rich crusts found in other areas of the world.

An official at the agency says it is surprising such a huge amount of mineral resources are found near Japan's main island of Honshu.

The official says the agency hopes to conduct research on the presence of more resources and the technical feasibility of marine resource development around Japan.