Cargo ships are being sunk here and some say it's all part of Russia's plan

Just after 1am on October 13, Commander Ivan Ivanov and his crew received a mayday call.

A Malta-flagged ship, the Chelsea 3, was in distress and sinking in the Black Sea, 40 kilometres from the Bulgarian port of Varna.

The vessel had been hit by a drone that had blown an enormous hole in its side, and it was taking on water.

Within minutes, Commander Ivanov's crew were heading out to sea to try and reach it.

"The weather was extremely bad. The sea was stormy, with a very strong wind, and the visibility was very low," Commander Ivanov told the ABC.

The Black Sea has become a deadly theatre of battle as merchant ships carrying things like grain, chemicals and oil are increasingly caught in the crossfire of Russia's war in Ukraine.

Both countries have ports on the Black Sea and have been hitting ships going to and from each other's docks since Moscow's full-scale invasion began.

But in the last few months, the attacks have taken on a whole new dimension, and sailors have become collateral damage.

At least 38 have been killed since June, making the Black Sea the deadliest waterway in the world, even surpassing the Strait of Hormuz.

When the crew arrived, the Chelsea 3 was already listing to the left hard and the choppy seas made it too dangerous to deploy a lifeboat.

The sailors had to jump from their sinking ship to the border patrol vessel to save themselves.

The ABC has gained rare access to embed with Bulgaria's border police, which act as the country's coast guard, undertaking search and rescue missions at sea.

Commander Ivanov has worked on the sea for 20 years, but the Chelsea 3 was the first sinking cargo ship in distress he had dealt with.

Just as his crew returned to port having saved all on board, he received a second mayday call. Another cargo ship, the HS Dignity, was sinking after also being hit by multiple drones.

But the worsening weather conditions would prove too dangerous. While five of the HS Dignity's crew were rescued after abandoning ship in a life boat, 11 remain missing.

Commander Bozhidar Penev, who has spent more than a decade on the water with Bulgaria's border police, was also on call that night.

"Definitely, the last few weeks have been tough for us," he told the ABC. 

"We have increased surveillance even further. We are always on high alert; we have multiple systems on board our ships to aid us in both daytime and night."

He said drones had made his job more dangerous.

"I am afraid of this [drones]; the possibility is always there. But if I am afraid or not, this is my job and when there is a task, I have to do it," he said.

The HS Dignity, flagged Guinea-Bissau, had been travelling from Algeria to Romania. Among those missing is Mohammed Mreish's 20-year-old cousin, Amjad.

"I feel they are alive, that's my real feeling. I have a big hope," he told the ABC last month.

He said Amjad's mother was distressed and would not have closure until her son was found.

Mohammed travelled from his home in Germany to Varna and spoke to some of the rescued sailors to find out more information about what happened.

"They woke up at the middle of the night ... and found the ship full of water, so they decided to run to the deck again to launch the lifeboat," he told the ABC.

He said they saw what they thought were two vessels coming towards them at speed and believed it was a rescue mission. They were actually sea drones.

Amjad was one of 14 Syrians among the crew. The country has been in turmoil and economically unstable as it rebuilds itself after former long-time dictator Bashar al-Assad was removed in 2024.

"They need money to live. They need money to survive. The situation in Syria is really not that good," Mohammed said.

"There's no work, no jobs, no money ... and the young people, they need to start their life. They need to work somewhere. And the only way is the sea."

'An attack is an attack'

Cargo ships traversing the Black Sea are forced to run the gauntlet of attacks by drones and missiles as Russia and Ukraine battle for control over the vital shipping corridor.

More than 500 ships have been hit within the waterway during the past year.

The number of sailors killed has risen fast because the nature of the strikes has changed from simply trying to disable ships to targeting their hulls and bridges in an attempt to sink them, a tactic known as "double tap".

The attacks are also now edging closer to Ukraine's neighbours and that means closer to territory falling under the North Atlantic Treaty Organisation (NATO).

Tomas Alexa from global maritime security agency Ambrey said he expected the attacks to continue.

"The Russians are testing the response, the unity of the Eastern flank," he said.

"They are monitoring and looking very closely what is the reaction of NATO, but also of the European Union to these attacks.

"In a way, we assess that it is likely that third attacks will take place in Romanian-Bulgarian waters simply because the glass was broken."

Romania and Bulgaria, which hug the western border of the Black Sea, are members of NATO alongside Turkey, which controls the access of ships through the Bosphorus Strait to the rest of the world.

During October, nine cargo ships were hit with drones within the exclusive economic zones of Bulgaria and Romania.

One was even inside the territorial waters of Romania.

The reason ships are travelling closer to shore now is the simple fact that wartime insurance premiums have skyrocketed. Sailing close to the shore comes with fewer risks.

The attacks are being seen as a major escalation and a test of NATO's collective security agreement, in which an attack on one member state is considered an attack on all.

The alliance, which aims to deter conflict, was formed after World War II between the United States, Canada, the United Kingdom and many European nations.

Bulgaria's former defence minister Todor Tagarev, who was in office when Russia launched its full-scale invasion of Ukraine, said the Kremlin's primary goal is to crush Kyiv's economy by cutting off sea trade.

However, it was also trying to test NATO's resolve, to see how far countries in the bloc will go to protect each other.

"One of the centres of gravity for Ukraine in this war is the European support to Ukraine," Mr Togarev said.

"When the Russians demonstrate how dangerous this is becoming for European countries, for European societies, then they hope that the support to the Ukrainian war effort will be diminished."

Despite being a member of NATO and the European Union, Bulgaria has historical ties with Russia that Mr Tagarev said was holding back a stronger response.

"They are really afraid to refer to Russia as an aggressor. And they do not want to have this as the basis for planning a response," Mr Tagarev said.

NATO's Article 5 (in which member states are obliged to come to each other's aid) has only been triggered once previously, after the September 11 attacks in New York.

Professor Kevin Rowlands from the Royal United Services Institute (RUSI) spent three decades in the British Navy and retired as a captain.

He said it was easier from a political standpoint to go to war for America in 2001 than what it would be against Russia.

"Some of the NATO allies are more closely aligned to Russia than others. Some would be pushing for this kind of declaration, others would be holding back, so it becomes very political," he said.

Beyond calling out Russia's actions, Europe is grappling with how to deter the attacks and often resorts to labelling it "hybrid" or "shadow" warfare.

"An attack is an attack. If two countries are at war, let's call it for what it is, it's a war," Professor Rowlands said.

NATO has sent reinforcements to the Black Sea including more patrol planes, radar systems and intelligence officers.

But with the frontline in Ukraine practically at a standstill, analysts believe Moscow is likely to continue hitting ships that fuel Kyiv's economy.