The explosion as a Japanese torpedo struck under the depot ferry HMAS Kuttabul was intense, throwing portions of the shattered hull high into the sky and killing twenty-one men.

Unfortunately for its crew, the boat became fouled inside the still incomplete anti-submarine boom net laid across the harbour between Georges Head and Green Point near Watsons Bay.

[1] Midget A (according to the Allied order of identification) from I-24, commanded by Sub Lieutenant Ban with Petty Officer Ashibe, next entered the harbour and followed a Manly ferry through the boom defences.

The submarine ran up the harbour unobserved for an hour until at 10:52 pm when it was spotted near the Garden Island Naval Base, across from the Sydney central business district.

Now in a position 180 metres (200 yd) off Garden Island and apparently proceeding in a direction towards the Sydney Harbour Bridge, USS Chicago opened up with its five-inch guns and oerlikons.

Recent research by historian Peter Grose indicates that one of Chicago's crew even emptied the magazine of his .45 automatic pistol at M24's conning tower.

[1] The historical events have been well covered (e.g. Jenkins, 1992, Carruthers, 2006 and most recently by Grose, 2007), however it is still unclear why it took Ban and Ashibe another hour-and-a-half to run the 1,600 metres; 0.87 nautical miles (1 mi) from just west of Chicago to an attack position off its eastern-pointing stern.

The wreck was found approximately 5 kilometres (2.7 nmi; 3.1 mi) offshore from Bungan Head near Newport in over fifty metres (one hundred and sixty feet) of water.

Details of the discovery were broadcast on the Nine Network's 60 Minutes television program on 26 November 2006, and the announcement confirmed by the Minister for Planning, in conjunction with relevant Commonwealth government agencies on 1 December 2006.

[1] A No-entry Protected Zone was declared around the fragile wreck site on 1 December 2006 by the Federal Minister for the Environment, under the Commonwealth Historic Shipwrecks Act 1976.

The building program accelerated from 1938 onwards, when some fifty Type A boats were built at Ourazaki and Kure naval yards, the majority between 1940–2 (number sequence Ha-3 to Ha-52).

The upper tube has been torn off the vessel back to a substantial bulkhead and lies almost totally buried in sand adjacent to the bow on the port quarter.

Just forward of the stern, the topside manhole that gave access to the motor room appears lost to corrosion, creating a minor cavity in the hull plating.

This sand has apparently entered the space as a result of significant damage to the upper conning tower structure and the aft battery room openings, mentioned above.

Only the pressure dome and periscope standard of the aft portion of the conning tower survives intact, however the damage has created an opening of approximately 50 centimetres (20 in) in diameter.

[1] The original signature of the external conning tower plating is clearly visible on the upper surfaces of the wreck, marked by the angle iron framework where it was attached to the casing.

Apart from some abrasion to the upper hull surfaces caused by fishing nets, and perhaps some modern impact by an anchor or some other foreign object, there is no significant damage to the casing.

Running along the forward centreline of the casing, the original hydraulic line that extended to the bow torpedo cap release has been torn off and lies across the hull to port.

What is clear is that the protective cage around the torpedo tubes has been stripped from the hull, including the forward net-cutting gear and serrated jump cable that ran back to the conning tower.

[7] The M24 is of heritage significance to Australia and Japan, and is the only Japanese midget submarine wreck located in Australian waters (the remains of two others from the Sydney attack having been recovered).

In regard to the three submarines that attacked Sydney Harbour, M24 is the only one retained in its original battle context and undisturbed, apart from the obvious effects of net entrapment and corrosion activity.

[1] M24 Japanese Midget Submarine wreck site was listed on the New South Wales State Heritage Register on 7 December 2007, having satisfied the following criteria.

While the raid was militarily of limited success, it highlighted the vulnerability of important Allied naval station and repair facilities, and ships, to long-range submarine attack.

The sale of artefacts from and regional tours of one of the captured midget submarines played a major role in raising awareness and in generating financial contributions to the War Bonds Scheme.

The final fate of M24 and its brave crew of Sub Lieutenant Ban and Petty Officer Ashibe became one of Australia's enduring naval mysteries until the site was discovered by recreational divers in November 2006.

The M24 wreck site provides an opportunity to document and analyse the form, fit-out and construction of a Japanese Type A midget submarine from World War II.

Aesthetically the M24 represents a rare opportunity to see a historically-significant submarine wreck site in its underwater setting, providing scope for world-class photographic documentation and potential controlled visitation by the recreational community.

The site and artefacts have the potential to contribute to technical studies of the construction, fit-out and materials used in Japanese midget submarines of World War II.

The site has potential to contribute to studies of corrosion activity of steel shipwrecks in coastal marine environments, and the effects of localised environmental factors.

As an example of the boats deployed in major war theatres such as Pearl Harbor, Sydney and Madagascar, the M24 wreck has the potential to document the main attributes of the class and various design additions.