An underwater volcano is thought to be behind a huge column of steam above the Pacific Ocean, Japanese officials say.
An undersea volcano last erupted in the area in 1986
The coast guard sent helicopters to monitor the 1,000m (3,280ft) cloud, 1,120km (700 miles) south-east of Tokyo, and warned ships to stay away.
The team said the area around the site appeared to be red.
"It's highly likely that it's caused by an eruption of an underwater volcano," coast guard spokesman Shigeyuki Sato said, adding it had happened before.
Japanese troops stationed on the island of Iwo Jima first noticed the cloud of steam on Saturday.
Television footage showed white smoke billowing into the sky from the brick-red water.
"We suspect the undersea volcanic moves are becoming active," said another coast guard official.
Further investigations will continue on Monday.
An undersea volcano last erupted in 1986 for three days in the area.