The 1,300-tonne KRI Nanggala 402 was a Type 209 diesel-electric attack submarine. Its construction began in 1978 and it was delivered to Indonesia in October 1981.

The KRI Nanggala, which had undergone several upgrades, was participating in naval exercises off the coast of Bali when it requested permission to dive and contact was then lost, authorities said.

There were 53 crew aboard the vessel, which was believed to be in waters about 700 metres (2,300 feet) deep.

The KRI Nanggala was refitted in 1989 in Germany and then in 2012 in South Korea, with part of its structure replaced as well as upgrades made to the propulsion, sonar and weapons systems.

Indonesia's navy possesses another submarine of the same model, the KRI Cakra, along with three others of different Type 209 models that were built more recently in South Korea and Indonesia, according to Janes, which specialises in military information.

In 1993 Indonesia also acquired 39 used ships from the former East German navy.

Developed in the 1960s to replace WWII-era vessels, the Type 209 was never used by Germany but enjoyed success as an export with 61 sold to over a dozen countries including Greece, India and Turkey.

Argentina deployed a Type 209 during the Falklands War against Britain.

Egypt received one in April 2020 that was built in the German shipyard where the vessel was developed and which is now owned by industrial giant Thyssenkrupp.

The company says on its website that the Type 209 was inspired by the coastal post-war submarines of the German navy, but enlarged to be able to operate in deeper waters and carry more equipment.

It said the vessel is "the top-selling non-nuclear submarine in the Western World".

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THYSSENKRUPP