What We Know So Far about the Sunken Indonesian Submarine KRI Nanggala

"You're dealing with remotely operated vehicles, that are not necessarily having the complexity and ability of a hand to take control of body parts."

He pointed out the Russian nuclear submarine, the Kursk was raised in 2001. The sub, which weighed about 20,000 tons, was lifted from a depth of just over 100 meters, but it took over a year to bring it up.

In contrast, the KRI Nanggala 402 is much lighter, at 1,400 tons, but also much deeper, at more than 800 meters.

"But even to lift a 1,400-ton submarine from the seabed requires a lot of logistics," he said.

"You've got to get the right ships in place, you then have to get all the way down to the seabed, and 840 meters is a long way down.

"You then have to get some form of flotation on the submarine, which would usually be things like diesel-filled bags that can then provide buoyancy — air is useless at that depth."

Tributes flow for sub on 'eternal patrol'

Indonesia's President Joko Widodo expressed his condolences to all the families.

Online, there has been an outpouring for the victims of the Nanggala – named after a divine weapon from traditional Indonesian puppetry, which, according to legend, can melt mountains and split the ocean.

Social media users have referred to the submarine as now being "on eternal patrol."