On a sunny day off the southern coast of Greece last week, two aging tankers nestled next to each other while one pumped oil to the other. As far as global satellite tracking systems could tell, it never happened.

The deviation between real and electronic locations — measured in this case at over four miles — wasn’t a glitch, but a deliberate deception that’s part of a sophisticated system to keep sanctioned Russian fuel flowing, often at prices that are higher than western powers would like.

The practice of giving fake coordinates to the automatic identification system, known as AIS, is called spoofing. It muddies understanding of where cargoes come from, soothing nervy buyers trying to conceal dealings with Russia following international measures to punish the country over its invasion of Ukraine.

The 800-foot Turba was one of the tankers engaged in the secretive maneuvers just a few miles from a pretty Greek coastal town on Sept. 19, according to Bloomberg observations and satellite data. Built 26 years ago — meaning it would normally have been scrapped by now — the vessel has become a potent symbol of Russia’s efforts to maintain its petroleum trade.

Back in May, the Turba drew regulatory ire due to its age, poor inspections record, mysterious ownership details and for flying the flag of Cameroon, a nation blacklisted by an international organization promoting safe shipping.

Following Group of Seven sanctions, Russia amassed a huge fleet of shadow tankers — often older ships like the Turba. The restrictions barred European Union member states from purchasing Russian petroleum and simultaneously prevented the bloc’s companies, especially insurers and tanker firms, from aiding in transportation. But once those vessels are in international waters — like off the coast of Greece — the scope to control the activity diminishes.

Estimates vary, but at least half of Russia’s oil is thought to flow out through shadow-fleet tankers like the Turba.

Read More: Russia Relies on Europe to Ship Its Oil Despite Price Cap Breach

Spoofing is a well-known practice for cargoes of oil where there’s a desire to conceal activity. While it’s frowned upon by regulators — and ships aren’t allowed into European ports if they do it — it is entirely legal in international waters, and companies offer the service openly on the internet.

The G-7 granted exemptions for handling Russian oil and fuel traded at or below fixed levels — $60 a barrel for crude and $100 for premium refined fuels like gasoline and diesel. However, soaring prices have pushed much of the nation’s petroleum above those thresholds, meaning that companies currently engaging in the service would raise questions about sanctions violations.

To spoof, somebody, somewhere would have a piece of hardware that sends out fake location signals while the vessel’s actual transponder is switched off by its crew.

Last week, the Turba’s false signals were captured and monitored on numerous well-known tracking systems, including one hosted by Bloomberg. It appeared to be miles from the Simba, another relatively old rust-covered tanker, but in reality, they were side by side in the Laconian Gulf, Bloomberg observed while visiting the area to monitor shipping activity.

“This kind of opaque transfer so close to European shores implies that the financial transaction for the cargo is above the price cap,” said Samir Madani, co-founder of TankerTrackers.com Inc., which followed the same shipment and also traced the original cargo back to Russia.

Attempts to reach the owners of the Turba and the Simba were unsuccessful. Three international shipping databases offer no means of contacting the owners or operators, and four brokers contacted by Bloomberg also didn’t have details.

The Laconian Gulf, about 110 miles southwest of Athens, has emerged as an important shipping hub for Moscow following sanctions. Other sites include Ceuta, a Spanish exclave on the Moroccan coast, and sometimes even deep in the Atlantic Ocean.

Under the practice, tankers would transfer Russia-origin cargoes from one ship to another. The precise reasons for any individual transfer are never clear, but the moves add a degree of separation and obfuscation for those ultimately purchasing the consignments. The switching had appeared to die down recently — especially near Ceuta — after Russia scaled back its oil and fuel exports to drive up prices.

Read More: Why Russia’s Ban on Diesel Exports Will Last Weeks Not Months

But Bloomberg’s observations show that the Laconian Gulf remains important. On Tuesday last week, three ship-to-ship transfers were taking place — all apparently involving refined fuels rather than crude — and about a dozen tankers were waiting.

The Simba had collected Russian-origin fuel from another vessel off the coast of Romania earlier this month, according to satellite imagery gathered and inspected by TankerTrackers.com. It then sailed via the Bosphorus for its rendezvous with the Turba. Kpler, another analytics firm, said the cargo was oil products.

While Greek firms aren’t permitted to provide services that help Russia transport oil, local authorities have no power to police the activity that goes on in the Laconian Gulf because cargo transfers take place in international waters, which start just six miles from the coast there.

Greece doesn’t publish information about tanker movements in the area and it’s unclear if it monitors the activity. Greek authorities weren’t immediately available to comment.

In addition to concerns about evading sanctions, the high-seas oil and fuel transfers pose a risk to marine environments as well as local tourism and fishing industries because of the older ships involved.

The switching between the Turba and the Simba took place about four miles from Greece’s territorial waters and about 17 miles from Gytheio, a popular seaside destination. The coastline is also an important nesting area for the loggerhead sea turtle.

In April, the International Maritime Organization singled out the Turba as an example of the pollution risk posed by shadow fleets. The UN agency also criticized a growing practice of ship-to-ship transfers and of vessels turning off transponders to conceal their activity.

--With assistance from Julian Lee, Angus Bennett and Alex Longley.