For 64 years the remains of two young Japanese sailors have lain in their undiscovered steel coffin — a midget submarine — not far off Sydney’s crowded northern beaches.

The amateur divers who found the M24 have opened up a debate about what should be done with the remains of Sub-Lieutenant Katsuhisa Ban and his navigator, Petty Officer Mamoru Ashibe. Even their families cannot agree on whether they should be raised and returned to Japan.

The pair launched a deadly attack on the night of May 31, 1942, when they slipped into Sydney harbour and fired their torpedoes at the USS Chicago, a moored cruiser. They missed and instead struck HMAS Kuttabul, an old barracks ferry, killing 21 Australian navy ratings and injuring scores more. The attack caused thousands of women and children to flee Sydney and sparked fear of a Japanese invasion.

The discovery has been made at a sensitive time; Japan and Australia are preparing to increase defence co-operation.

The brother of Petty Officer Ashibe said from Japan that he was “filled with hope” that the M24 would be raised. Itsuo Ashibe said: “As a member of a bereaved family — and someone who lost four brothers during the war — I pray that it will happen.” Kazutomo Ban, brother of the M24’s commander, said: “I feel relieved that we now know the exact place he died. Now, I think, we should leave him to rest in peace.”

If the M24 is raised, the Australian Navy will need to decide whether to provide full military honours for the submariners. While the M24 did the most damage on the night it entered Sydney Harbour, it was not alone. Two other midget submarines accompanied it. They were both destroyed and recovered. Their four crewmen were secretly given full military honours at their burial in Sydney in a ceremony arranged by Rear Admiral GC Muirhead-Gould, Sydney’s naval chief, an Englishman who was deeply impressed with the bravery of the Japanese submariners. He was later to say: “It must take courage of the very highest order to go out in a thing like that steel coffin. How many of us are really prepared to make one thousandth of the sacrifice that these men made?”

While Australian and Japanese officials have begun discussions about what should be done, a diving exclusion zone has been put around the site.

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