What is the mysterious Yantar 'spy ship' - and is it mapping Britain's undersea cables?

A Russian 'spy ship' long suspected of surveilling the UK's crucial network of undersea cables has been spotted on the edge of our waters once again.

The Russians claim the Yantar is a research vessel, but it is operated by a branch of the Kremlin's armed forces - and its intentions are murky. "This is the second time in a year that it's entered UK waters," defence secretary John Healey said on Wednesday.

The UK is connected to Europe - and beyond - by networks of cables that run along the sea bed. In a Sky News report earlier this year, we examined whether they are vulnerable to sabotage.

So what is the Yantar ship, and what is it up to? A 'research' vessel Labelled as a "research" or "survey" ship, the Yantar flies under the Russian flag. It is operated by the Main Directorate of Deep-Sea Research - a secretive branch of Russia's armed forces.

The Yantar is around 34m (112ft) long and features various antennae and detection equipment onboard. It is reportedly designed to act as a mothership to mini-submarines, which can then examine environments deep beneath the surface of the ocean.

It's believed that two submersibles hosted on the Yantar can reach depths of 20,000ft - that's almost twice as deep as the wreck of the Titanic. In an era of hybrid warfare and deniability, it's notable that these capabilities are useful for both civilian and military applications.

Is the Yantar a spy ship? Many outside observers have long said so, including the UK ministry of defence. Russia has, of course, not said that the ship is being used for anything other than legitimate maritime research.

However, others point to its pattern of behaviour and where it tends to hang out. As John Healey put it earlier this year - when the Yantar entered UK waters - "this is a Russian spy ship used for gathering intelligence and mapping the UK's critical underwater infrastructure".

And while the Yantar complied with international rules of navigation, this wasn't even the first time it had been detected near Britain's subsea installations. Are Britain's undersea cables vulnerable? Experts believe Russia has spent recent years covertly mapping undersea cables in the West - some of which are military and whose locations are not public knowledge.

In September, MPs and peers on the National Security Strategy Committee concluded that the government is "too timid" in defending the UK's undersea cables. Read more:Why UK's undersea cables are vulnerableUK plan to defend from invasion moving at 'glacial' pace Dr Sidharth Kaushal, an expert in maritime technology at the RUSI thinktank, previously told Sky News that while there is a degree of redundancy in the undersea cables that serve the UK, the pipelines that bring gas to British homes are perhaps more vulnerable.

"I think in some ways the pipeline network is far more fragile because there we are more reliant on a handful of critical pipelines," he said..