The galleon was believed to be carrying cargo worth as much as US $80 million in modern terms, according to a BBC news report, but no other trace of the ship has been found before.

The cannonball was found by Ian McCann, an Australian researcher from the University of New England, during a dive to 40m off Iwawada.

“We were in dark, murky waters,” said Dr Jun Kimura from Tokai University, leader of the first scientific mission to search for the San Francisco.

“Ian just saw an unusual shape on the sandy bed – he recovered it but then we had to go back to the surface, as our air had nearly run out.”

The cannonball resembled others found from Spanish traders in the Philippines, and the team were “almost sure” that it came from the San Francisco, but hoped to confirm this through chemical analysis.

Timber found nearby was also believed to be related to the wreck.

“It is the only Spanish Manila galleon that has not been plundered by treasure-hunters,” McCann told the BBC. 

The governor of the Philippines, Don Rodrigo de Vivero Velasco, was on board the San Francisco when a storm drove her onto reefs and broke her up.

He and hundreds of other survivors eventually sailed back to Mexico on the first Western-style ship ever to be built in Japan.

Japanese representatives who accompanied them were considered to be the first ever to cross the Pacific, and the incident and its positive outcome established firm links between Spain and Japan.

The recent underwater expedition was funded by the Japanese government, with further exploration planned for early 2018.

Divernet - The Biggest Online Resource for Scuba Divers