For much of the past three years, tankers carrying Russian crude oil – usually in violation of western embargo – skirted western sanctions and oversight by engaged in so-called Ship-to-Ship (STS) transfers somewhere in the open sea far from prying eyes and even further from hostile coast guard supervision.

The practice, usually carried out in secret with digital tracking beacons switched off or falsified, can help to obscure the origins of the oil, helping to beat sanctions. It also creates another layer of separation between the buyers and sellers of cargoes.

Of course, to keep STS as a viable option, the places where it takes place have to change periodically. And as Bloomberg reports, the secret switching of Russian fuel cargoes between tankers at sea has migrated to new hotspots off the coast of Greece after the European country used naval drills to try and block the activity in one location.

About 1 million barrels a month of diesel, fuel oil and other petroleum products has been getting flipped near the islands of Lesbos and Chios in the Aegean Sea, according to data from analytics firm Vortexa.