On 26 January, staff in an office in Mumbai received an urgent e-mail from a crew member aboard a tanker off the coast of Singapore.
The email, purportedly written on behalf of five colleagues aboard the tanker sailing under the name Beeta, contained a litany of complaints: crew members, it was alleged, had not been paid and were being treated like animals; and provisions were running low.
The staff in Mumbai worked for the International Transport Workers Federation (ITF), the world's leading organisation representing seafarers, and were used to dealing with complaints from all corners of the globe.
But what caught their eye was the fact that the emails hadn't just been copied to multiple ITF offices, but also to sanctions enforcement bodies in several countries.
The vessel is sanctioned and blacklisted, the sailor wrote.
He said he had discovered that the vessel calling itself the Beeta was in fact an American-sanctioned tanker called the Gale.
The sailor and his colleagues were desperate to leave.
I've been at sea for many years, he told the BBC when we contacted him. I know what's right and wrong.
Inadvertently, the crew member had found himself involved in a problem at the heart of some of today's most contentious geopolitical issues: a surging number of tankers, transporting Russian and Iranian oil, operating outside maritime rules, using a variety of methods to conceal their identities.
This shadow fleet, as it's known, is growing fast.
Estimates vary, but the latest data from the monitoring group TankerTrackers.com says the fleet currently consists of 1,468 vessels, roughly triple its size at the time of Russia's full-scale invasion of Ukraine four years ago.
That's about 18-19% of the total internationally trading tanker fleet, carrying approximately 17% of all seaborne crude, says Michelle Wiese Bockman, a senior maritime intelligence analyst at Windward AI.
The phenomenon that first emerged in the 2010s, as North Korea and Iran sought to evade international sanctions, has proliferated, leaving western governments scrambling to keep up.
But what can those governments do when countries circumvent or flout the rules to export oil and use the profits to fuel engines of war or repression?
Definitions vary - as do names: it's also been called the dark or ghost fleet - but shadow vessels generally exhibit some or all of a particular set of characteristics.
They tend to be old, averaging an age of around 20 years, and are often poorly maintained. Details of ownership and management are deliberately opaque - names, identification numbers and flags are frequently changed. Insurance is below Western standards and sometimes fake.
Vessels frequently switch off or manipulate their automatic identification system (AIS), making them harder to track.
Seamen, recruited on contracts that typically last six to nine months, don't always know what they're getting into.
I didn't understand what shadow fleet vessels actually were, Russian engineer Denis told us, after serving aboard an EU- and UK-sanctioned tanker, Serena, for several months last year.
Denis says he realized the Serena was under sanctions only when he went aboard. But like many seamen, he says he needed the work.
Over the course of months spent sailing between Russia and China, the reality of life aboard a shadow vessel became increasingly apparent.
Things began to deteriorate very quickly because no spare parts were sent, he said. The radar on the bridge hadn't been working for over a year, the life-saving equipment wasn't working and there were numerous faulty fire sensors.
Deep within the murky recesses of the shadow fleet also lurks a category experts have dubbed zombie ships - vessels that steal the identity of dead ships in order to hide their true identity.
After being sanctioned, a zombie ship often disappears from tracking systems, later reappearing under a new name, using a stolen nine-digit International Maritime Organisation (IMO) number from a scrapped or decommissioned vessel.
For instance, the Gale is one such ship, reportedly flying a false Gambian flag, which has assumed multiple identities since being sanctioned by the US last year for its involvement in the export of sanctioned Iranian oil.
Such practices, coupled with the knowledge that Russian and Iranian oil revenues continue to fuel conflict in Ukraine and the Middle East, are driving an urgent debate in western capitals about how to respond.
While proactive measures have been taken, significant obstacles remain in the enforcement of sanctions, particularly with the expansive and elusive nature of these shadow fleets.
Ultimately, the sheer number of shadow vessels poses a simple problem of resources. Whether international efforts can effectively counteract the growing menace of these maritime miscreants will shape the geopolitical landscape in the years to come.
