The Thunder was the world's most notorious poaching ship.

It plundered Antarctic waters for years, raking in millions in illegal profits from its hauls of Patagonian toothfish — known as 'white gold' in fishing circles.

And it didn't go down without a fight.

Its round-the-world bid to evade capture would go down as the longest maritime chase in history — and expose a murky web of crime and corruption on land.

"It's a thriller, a real-life thriller," Kjetil Saeter, a Norwegian investigative journalist who helped track the ship's kingpin, tells RN.

Sorry, this audio has expired Kjetil Saeter tells RN's Geraldine Doogue the story of the round-the-world pursuit

It's also, he says, "organised crime at its worst".

"The perpetrators behind the whole flag state, the ship agents, the lawyers, the tax havens … it's the same as you see in human trafficking, in gun smuggling," Saeter says.

"It's the same kind of networks operating, they use the same schemes."

LtoR: Kjetil Saeter and Eskil Engdal travelled to five countries to investigate the money and corruption behind illegal fishing. ( Supplied: Scribe Publications )

The Thunder was part of a group of rogue ships known as the Bandit 6.

It had operated under several different names and flags, and was wanted by Interpol, which estimated it had earned $US60 million for its unknown owners.

It was also in the sights of anti-whaling group Sea Shepherd, which had decided to go after the illegal fishing ring.

In December 2014, Sea Shepherd's Bob Barker found the Thunder in the icy, inhospitable waters off Antarctica.

It hoped to trail the ship to port, where authorities would be waiting.

For the next 110 days — and some 16,000 kilometres — the Bob Barker gave chase, and the Thunder tried to lose its shadow.

It became a battle of wits between the two captains, who communicated over radio, each trying to convince the other to give up.

The Thunder was wanted by Interpol ( Supplied: Sea Shepherd/Simon Ager )

As the high seas chase unfolded, Saeter and a colleague, Eskil Engdal, started asking some questions.

"What really triggered us was an Interpol notice of a vessel. Why do you publish a notice on a fishing vessel?" Saeter says.

"You don't arrest the getaway car in a bank robbery, you arrest the bank robber.

"So the mystery here [was] who was actually on board the Thunder, who are these guys behind these balaclava masks, and who were the guys making money on this?"

After an investigation spanning five continents and "some not-so-nice people", the journalists found their answer.

"In Thunder's case they are living in Galicia, north-western Spain," Saeter says.

The pair also uncovered an international habit of turning a blind eye.

A Patagonian toothfish pictured in a net left by the Thunder vessel. ( Supplied: Sea Shepherd/Jeff Wirth )

"The issue with illegal fishing [is that] 70 per cent of the Earth's surface is international waters, so Australia doesn't own it, Norway doesn't own it," he says.

That makes it harder for any country to really police them —especially when pirate vessels have moved to the icy edge of Antarctica.

"So if you have a crew from Indonesia and you have your vessel flagged in Mongolia and it's owned from Panama, and then you have countries that don't have a navy to go down there and arrest them, they don't really care," Saeter says.

A suspicious sinking

The sea chase ended some 16,000 kilometres from where it had begun.

In April 2015, the Thunder came to a complete stop off the western coast of Africa.

"The crew came out with their life jackets on and threw a ladder over the side," Peter Hammarstedt, captain of the Bob Barker, told The Guardian at the time.

"I radioed the captain to ask if they were in distress and he said 'yes, we're sinking'."

The Thunder lists dramatically to the starboard side as it takes on water. ( Supplied: Sea Shepherd/Simon Ager )

The captain of the Thunder claimed he was hit by a passing cargo ship, but others believe he deliberately scuttled his vessel.

"You know, you run out of gas and you have evidence on board your vessel, you flush it down the toilet as a cocaine dealer would do, at the last minute," Saeter says.

The Bob Barker's captain agrees.

"It was suspicious that all the hatches and doors were left open, some tied open," Captain Hammarstedt said at the time.

"That's counterintuitive to keeping a vessel afloat. They opened it up so it would sink, the engine room was completely filled with water.

"It's been a siege situation — who would run out of fuel, food and patience first. They obviously ran out of patience first."

As the Thunder went down, the Bob Barker — and two other ships that had joined the chase — rescued and detained its crew, many of whom had been working in slave-like conditions.

'A victory for the oceans'

The captain and fishing master of the Thunder wait to be taken away by Sao Tome authorities. ( Supplied: Sea Shepherd/Jeff Wirth )

The Thunder's renegade captain and two crew members were later convicted on several charges tied to illegal fishing.

They were jailed for around three years, and collectively fined more than $US17 million.

The attorney-general of Sao Tome and Principe, the island state where the trial took place, hailed the verdict as "a victory for the oceans".

Saeter and Engdal have now authored a book, Catching Thunder, about the record-breaking chase and their investigation.

Asked whether the good guys won, Sater says yes — in a way.

"The man behind the Thunder, he's gotten some fines now in Spain. He's not in jail, I don't think he will go to jail because of laws and this having happened in international waters," he says.

"We don't believe there are any vessels down there now, but of course this story about the Patagonian toothfish, about Antarctica, is just one small story about illegal fishing around the world."

The problem of illegal fishing, he says, isn't so easily solved.

"You have all these tax havens still, you have all these strange flag states that will sell their sovereignty to a vessel like this," Saeter says.

"So yes, we can be positive but there's a long way to go I think."