TOKYO >> Two border islands in Japan that have served as reference points to determine the nation’s territorial waters might not exist anymore.

The two islets, Seppu-­Minami-Kojima and Shio- kubi­-Misaki-Minami-Kojima, are in waters off Hokkaido and were about 1,000 square feet each.

But the islets are not visible in satellite photos, a problem that has occurred with other border islands; there are more than 480 in Japan.

Seppu-Minami-Kojima, about 720 feet off the coast of Niikappu, a town in southern Hokkaido, might have disappeared due to changes in topography after a powerful 2018 earthquake.

Shiokubi-Misaki-Minami-­Kojima, about 330 feet off the coast of Hakodate, is believed to have disappeared during work on a mainland embankment near the islet.

They are among eight islets that cannot be traced.

Remote border islands serve as reference points to determine Japan’s territorial waters and exclusive economic zone.

As of last year there were 484 remote border islands across Japan, some of which are inhabited. In 2017 the government had completed all procedures for nationalizing and naming the islands, steps toward securing maritime rights and safeguarding the nation’s territory.

Since the existence of the islets cannot be confirmed, the Japan Coast Guard, the Geospatial Information Authority of Japan and other agencies are conducting studies.

One islet in the Sea of Okhotsk, off the village of Sarufutsu in Hokkaido, might have broken apart due to erosion by waves and ice floes.

Five others cannot be found at the locations indicated on a GSI map. But the accuracy of the map is in question. The Coast Guard and GSI plan to verify the locations of the border islands using aircraft and other means, to determine whether the map should be modified.

The government will follow up by creating a database of border islands to boost the monitoring system.