YOMITAN, Okinawa — A large plume of white smoke spotted about 875 yards off the beach at Torii Station turned out to be a rare spontaneous detonation of a World War II-era explosive, a Japan Maritime Self-Defense Force spokesman said Friday.

“The bomb just happened to wash up onto the reef by the forces of nature, exposing the content to air, which led to ignition,” 1st Lt. Masakazu Karimata, spokesman for the JMSDF base at White Beach, said of the Wednesday phosphorus-bomb explosion. “This is the first such case in our recorded ordnance disposal operations on Okinawa since the reversion to Japan in 1972.”

A second unexploded bomb was discovered Thursday during a search of the marine area.

The beach, on Okinawa’s central west coast, was one of the main landing sites U.S. troops used during the Battle of Okinawa in April 1945. The discovery of unexploded ordnance dating from the battle, referred to locally as the “Typhoon of Steel,” is a common occurrence.

During a search of the area, a JMSDF explosive ordnance disposal team also discovered and removed an unexploded 105 mm artillery shell about 65 feet away from where the phosphorous bomb exploded, according to JMSDF Lt. Commander Akihito Matsunaga. Because of deterioration, it was unclear whether the ordnance was Japanese or American.

The U.S. Army is cooperating in the investigation, said Chip Steitz, spokesman for the 10th Support Group.

“It is not unusual to find such rubble of war in Yomitan,” said Hitoshi Yonaha, spokesman for Yomitan Village. “There are probably more various things to surface around here. Experts tell us it will take another 70 years to dispose all of such remnants of the war.”

Yonaha urged beachgoers to be on the lookout for such items whenever they enter the water.

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