Here’s what we can gather about the US nuclear attack submarine collision in the South China Sea

A nuclear-powered submarine colliding with an unknown “object” under one of the world’s busiest shipping lanes is rarely good news – but is the incident cause for concern?

Submarine incidents are usually shrouded in secrecy, given how critical these sophisticated (and expensive) pieces of equipment are in military operations.

We’ve talked to defence and security experts to help clarify what we know, what we don’t know and what we may never know.

What could the unknown object possibly be?

Vipin Narang, a professor of nuclear security and political science at MIT, says we shouldn’t hold our breath when it comes to the USS Connecticut.

“It’s totally unclear what hit it,” he said.

When asked about the possibility of it hitting an enemy submarine, he said he doubts we will ever know.

“We likely won’t know for sure for a while, if ever, on this one,” Dr Narang said.

“Not sure why we heard about this one — sometimes the damage is extensive enough that it’s hard to hide,” Dr Narang said.

But what have submarines collided with in the past, and can that give us any clues?

Peter Dean, Director of the University of Western Australia’s Defence and Security Institute, told the ABC there were a few possibilities.

“The submarines run aground, hitting something on the bottom. They are relying on their sonar, so it’s difficult to get an accurate picture of the underwater environment. Large wildlife, like whales, have been a problem, and of course, other submarines,” Profesor Dean said.

Euan Graham, a senior fellow for Asia-Pacific Security with the International Institute of Security Studies, largely agrees.

“They usually have good topographic knowledge of the areas they are patrolling, but there is always a risk of hitting a loose shipping container or other submerged object,” he said.

The South China Sea, with its busy shipping lanes and fishing areas, is particularly risky for this, he added, especially when operating at shallow depth.

The number of shipping containers that fall off ships and sink to the bottom is astonishing, Professor Dean said.

How rare are underwater submarine collisions?

The experts say such incidents are unusual but not unheard of.

There have been a number of such collisions in recent years, says Professor Dean.

In 2009, British submarine HMS Vanguard collided with French submarine Le Triomphant in the Atlantic Ocean while carrying nuclear missiles, in what the New York Times described as a “freak accident”.

In 2005, BBC reported an American sailor died after his nuclear submarine – the USS San Francisco – ran aground near Guam.

Dr Graham said the fact that the USS Connecticut went to Guam suggests it suffered damage.

“The fact that 11 injuries were reported and that the submarine was forced to transit to Guam on the surface suggests that the damage could be serious,” Dr Graham said.

The US has said the submarine’s nuclear propulsion system was unaffected by the collision and was fully operational.

How does monitoring equipment miss objects large enough to cause damage?

Submarines use sonar rather than radar to detect objects around them when underwater.

But they don’t use it constantly.

“They use sonar in passive mode or listening mode, they have sensors all around the sub, and an acoustic tail with sensors all over it. They do have active sonar that send out pings, which gives away your position, so they tend not to use that. Sometimes even with the best equipment, it’s very hard to hear things,” Professor Dean said.

Dr Graham said the South China Sea in particular was not conducive to listening.

“The South China Sea is notoriously noisy, so acoustic detection is more challenging than in quieter seas,” he said.

What is the worst-case scenario?

The USS Connecticut is a nuclear-powered submarine with a nuclear reactor on board and the risk of explosion is “a worry”, said Professor Dean.

“We saw the Indonesian submarine that sank not long ago, we don’t know the cause of that because no-one survived. In the 2000s, the Russians lost a nuclear submarine in the Barents Sea which triggered an explosion,” he said.

“Anything with a nuclear reactor on board is a worry, you’re talking environmental damage as a result of that. What also tends to happen when submarines sink,” even without an explosion, “is that it’s rare for anyone to survive,” Professor Dean said.

Any significance of this incident occurring in the South China Sea?

The submarine collision has come at a time of heightened tensions in the Asia Pacific, with China and US relations worsening, and tensions between China and Taiwan heating up.

China has significantly increased the number of warplanes it sends into Taiwan’s Air Defence Identification Zone, in moves considered provocative by both Taiwan and the US.

But the South China Sea, as mentioned earlier, is home to one of the world’s busiest shipping lanes, and is also now a geopolitical hotspot as China and the US vie for primacy.

Professor Dean said it was “very, very unsurprising” for this incident to occur in that region.

“These things can happen anywhere, they’ve tended in the past to happen in European and other waters, but there are a lot more nuclear submarines operating now in the Indo-Pacific than in the past,” he said.