Japan's continental shelf expanded in July by 120,000 square kilometers, equivalent to about 30% of its total national land area, with the addition of an area in Pacific waters east of Chichijima, part of the Ogasawara island chain.

The expansion, achieved through continuous maritime surveys conducted by the Japan Coast Guard, has sparked optimism for potential resource development. The new region is believed to contain valuable deposits of rare metals, including cobalt.

The continental shelf forms part of a country's exclusive economic zone, where the country holds priority rights to resource development. Under the 1982 U.N. Convention on the Law of the Sea, a continental shelf can be extended if continuity is demonstrated on the basis of topographical and geological characteristics.

In response to the convention, the JCG began investigating Japan's continental shelf in 1983. After 25 years of maritime surveys, the JCG submitted an application to the U.N. Commission on the Limits of the Continental Shelf in 2008, seeking recognition for seven areas along Japan's EEZ in the Pacific Ocean.

In 2012, the U.N. commission recommended the extension of four out of the seven areas. Among them, the Shikoku Basin and south of the Okidaito Ridge were officially added to Japan's continental shelf in 2014.

The remaining two areas required coordination with the United States. Of them, the Ogasawara-Kaidai area was successfully agreed, leading to its inclusion in Japan's continental shelf in July this year.

Survey ships from the JCG's Hydrographic and Oceanographic Department have conducted surveys across the vast Pacific Ocean. The department traces its origins back to the hydrographic section established within the Navy Department of the Ministry of War in 1871, which was responsible for producing nautical charts of Japan's coastal areas.

The charts display numbers indicating water depth. In the past, depth was measured by weighted ropes lowered into the sea. Currently, multibeam echo sounders mounted on survey ships use sound waves to measure both depth and geological conditions. Additionally, aircraft and specialized equipment that emit sound waves from near the seabed are now employed for more precise measurements.

Two JCG survey ships — the 2,400-ton Takuyo, commissioned in 1983, and the 3,000-ton Shoyo, commissioned in 1998 — were deployed to investigate the newly extended continental shelf. Once deployed, the ships typically remained at sea for about a month, conducting surveys around the clock. They continuously made U-turns after covering set distances to ensure comprehensive data collection.

The seabed features mountains and cliffs, similar to land topography. During the surveys, the JCG vessels discovered a series of ridges known as mullions on the ocean floor, about 600 km southeast of Okinotorishima, Japan's southernmost island.

Measuring approximately 125 km long and 55 km wide, the mullions are about 10 times larger than previously discovered formations. The magnitude led the JCG and researchers to name it the "Godzilla Megamullion," after the famous fictional monster. The name was officially registered at an international conference in 2022.

Japan has also named several distinctive geographical features after renowned writers. These include Soseki Ridge, named after novelist Natsume Soseki, Ryunosuke Seamount, after novelist Ryunosuke Akutagawa, and Basho Seamount, after poet Matsuo Basho. According to JCG officials, Japan is considered one of the world's leading nations in geographical naming.

Japan is not alone in its efforts to extend its continental shelf. With other countries also conducting maritime surveys to secure resource development rights, Japan faces disputes with China and South Korea over the extension of the continental shelf in the East China Sea.

Starting in fiscal 2027, the JCG plans to commission two new survey ships with enhanced capabilities, reinforcing its commitment to maritime surveys. "We will continue to make steady efforts, even though it takes much time to achieve results," an official said.