Hope fades for the 53 crew members of a missing Indonesian submarine, as the search for the ship continues off Bali.

The Indonesian Navy has revealed that an oil spill at sea was spotted near the early diving position of the KRI Nanggala-402 before it disappeared.

Indonesia’s Defense Ministry said the navy lost contact with the vessel shortly after being cleared to dive, around 3 a.m. on Wednesday.

Previously, defense officials said they believed the vessel suffered an electrical failure during the dive, causing it to lose control and prevent it from resurfacing. They believe it may have sunk to a depth of 600 to 700 meters.

Frank Owen, an expert in underwater rescue, told the Guardian that this vessel was only designed to dive to a depth of less than half that.

“The submarine is designed to go about 250 yards, and they usually have double the safety margin, but when you start to descend below, who knows.”

“At that depth of water, they could still be in one piece, but there probably would have been a lot of buckling. If something fails, everything fails.

Owen said there were three likely outcomes for the subs and his crew.

“It’s either on the seabed or on the surface. Or it just works and it doesn’t know people are looking for it. This is one of the three options. “

“It could be a communication failure, but it’s been 24 hours since the submarine was lost, so it seems less and less likely.

He said it was unlikely the submarine was still in operation and had just lost communication.

“They had an exercise they were going to do, there was an expectation that they were going to communicate, but they weren’t communicating. So it’s either a communication problem or a submarine problem.

Several countries, including Australia, Singapore and India, responded to a request from Indonesia for assistance in locating the missing vessel.

“We have had good cooperation with Singapore and Australia in finding and dealing with training accidents,” said Air Force Marshal Hadi Tjahjanto.

Australian Defense Minister Peter Dutton said he called his counterpart, Prabowo Subianto, and offered Australia help.

Foreign Minister Marise Payne also said Australia would provide all the help it could, but said the underwater rescues were complex and difficult.

“The news of the missing submarine is deeply worrying. There are more than 50 submariners on the boat and the reports we heard overnight are clearly going to be deeply distressing for the families of these submariners and even for the Indonesian Navy.

“Australia has indicated and has been in contact with Minister Prabowo through Defense Minister Dutton that we will provide all possible assistance. There is no doubt that underwater search and rescue is very complex.

“This is not a submarine that Australia operates; our class of submarines is quite different, but whatever we can do, we are committed to doing it, and I think these submariners and their families really need all of our thoughts and prayers.

The Indonesian Navy said it has deployed all ships equipped with underwater devices to assist with the search and rescue mission.

The missing Type 209/1300 submarine was built at Howaldtswerke-Deutsche Werft in Germany in 1977 and was officially inducted into the Navy in 1981, making it one of the oldest submarines still active.

James Goldrick, retired rear admiral and naval historian, said the ship’s age could have been a contributing factor.

“Reaching 40 is really starting to push him. Almost no one uses such old submarines.

“The concern here is that not only is the boat approaching 40 years as the original boat, but the time since the last major refit is approaching the limit.”

The last time KRI Nanggala was remodeled was in 2012, in South Korea, where parts of its structure were replaced or improved.

Goldrick said rescue efforts would be extremely difficult, especially if the ship did indeed sink to the bottom of the sea.

“The problem is, if the sub is in an area that is too deep, there’s not much you can do about it if it has actually fallen. It is practically impossible to take them out at this depth.

“The seabed isn’t necessarily flat, and there’s a good chance there will be crevices and upwelling, so even though the submarine is still a whole hull, it can be very difficult to find.

“The submarine can very easily be wrong. The risk that the worst has happened is very high. “

Goldrick said even finding the vessel would be difficult, although the oil spill spotted near the last dive position gave an indication of where it was.

“The oil spill might give a hint, and with an understanding of the environmental conditions, prevailing currents and wind, you might be able to say, well, you can find a probable position, but it’s very difficult.

“So you have an area of ​​probability that you have to look into, and even in very deep water, a small area of ​​possibility can be very difficult.

“It’s not easy to bring down a submarine that has disappeared.”

He said many Australian submariners would watch the news with anxiety and it was still hard work.

“All Australian Navy submariners will be worried about them.

“It’s a very high risk job that requires constant professionalism and a crew that does everything right, all the time. And equipment that works. “