The Japanese oceanographic research vessel Mirai entered the Arctic Ocean in the small hours of Thursday local time, after passing through the Diomede Islands, located at the center of the Bering Strait, on Wednesday night.

Little Diomede Island, one of the two islands comprising the Diomedes, is U.S. territory, and the other, Ratmanov Island, is Russian territory. With the international date line running between the two islands, Little Diomede Island, located to the east of the date line, is also called "Yesterday Island," and Ratmanov Island, to the west, is nicknamed "Tomorrow Island."

Ratmanov Island, the larger of the two, is the easternmost point of Russia. The two islands are only 3.8 kilometers away from each other at their closest points, but there is a 20-hour time difference between them in summer. About 80 people live on Little Diomede Island.

Ice sometimes forms between the two islands during winter, making it the only place people can walk between the United States and Russia, according to the United States Geological Survey, a U.S. government agency. But such travel between the islands is not allowed.

To pass through the Bering Strait, the Mirai sailed in U.S. territorial waters, traveling about 8 km northeast of Little Diomede Island. The ocean was covered with fog as the ship approached the island, with the visibility falling below 100 meters.

The Mirai, operated by the Japan Agency for Marine-Earth Science and Technology, or JAMSTEC, left its home port of Sekinehama in the city of Mutsu, Aomori Prefecture, in late August.

It is on a month-long voyage to study environmental changes in the Arctic, which is believed to be warming faster than anywhere else on Earth.