Robert HARNEIS -TDO- (FRANCE)-  Indonesia has accepted an offer from China's PLA Navy for assistance in raising the wreck of the submarine KRI Nanggala, which disappeared without warning during an exercise dive on April 21. 

The Indonesian Navy confirmed on Saturday that three Chinese salvage and sub rescue vessels are under way to the location of the wreck site in the strategic Lombok Strait. 

A Chinese salvage mission is likely to be cheaper than Western alternatives. At the same time, a Chinese defense source told Global Times. The salvors will have an opportunity to "study the maritime military geography of the area where the submarine was wrecked, as well as expanding the international cooperation and influence of our navy in submarine rescue and salvage".

The cause of the sinking is not known, but senior Indonesian officials told The Straits Times that it is possible that the sub was dragged down below its crush depth by an internal wave. Lombok Strait is known for its internal waves, powerful subsurface currents driven by ocean layers and tides which can sweep through the water column in ocean channels. They are potentially deadly for submarines, as they can pull a layer of water (and its contents) down to great depths - or force it suddenly upwards towards the surface. The Strait of Gibraltar is also known for its strong internal waves; at least one submarine casualty in that region has been attributed to the phenomenon. 

KRI Nanggala was a Type 209 diesel-electric attack sub, one of many built by Howaldtswerke-Deutsche Werft (now ThyssenKrupp) in Kiel between the 1970s and the mid-2000s. The class was developed specifically for export, and more than 60 were built and sold to 13 nations, including two for Indonesia. KRI Nanggala was delivered in 1981, and underwent an overhaul and modernization at DSME in 2012. 

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