An unmanned research submersible scans fields of cobalt-rich crusts found on the surface of a underwater mountain, and its remote-controlled arms take samples of the crusts. (Provided by Japan Agency for Marine-Earth Science and Technology)

A vast geological deposit rich in rare and important metals has been found on the seabed off the Boso Peninsula in Chiba Prefecture.

The cobalt-rich crust forms around rocks on the seabed, and consists of iron, manganese and other rare metals such as platinum and cobalt that can be used in many industrial products.

The area is about 950 square kilometers, about 1.5 times the total area of Tokyo’s 23 wards, and could possibly allow Japan to secure rare metal resources much closer to home than usual.

A group of Japanese research institutions including the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) announced the discovery east of Tokyo on June 5.

A cobalt-rich crust region had previously been discovered near Minami-Torishima island, which is about 2,000 km off Honshu, Japan's main island.

It was believed that waters much closer to the coast would be an unlikely home to cobalt-rich crust as organic substances from the land would accumulate on the seabed instead.

The treasure trove of rare metals was found on an underwater mountain about 350 km east-southeast off the Boso Peninsula in the Pacific Ocean.

An unmanned research submersible captured the black minerals resembling asphalt that cover an expanse of the mountain slope between a depth of 1,500 and 5,500 meters. In some parts, the sampled crust was 13 centimeters thick, a rare size even by global standards.

“There are many places with a similar topography in waters closer to the Japanese coast,” said Katsuhiko Suzuki, project leader for JAMSTEC. “Further research is needed.”