Shipping has long been one of the most opaque of global industries. Now many operations in the oil sector, which accounts for nearly a third of all seaborne trade, have become still more secretive, following the West's imposition of sanctions on Russian oil. A new "dark fleet" of ageing tankers with obscure ownership, flying flags of convenience, has been formed to avoid the sanctions - and there has been a big increase in risky ship-to-ship transfers at sea, which make it easier to disguise the oil's origin. The Laconian Gulf in southern Greece is a major hub for such transfers, and locals now fear any accident could cause major environmental damage in an area which depends on tourism and fishing. Tim Whewell witnesses the operations close-up, and talks to campaigners who believe the influence of powerful shipowners makes local authorities turn a blind eye to possible dangers. Who is responsible for policing the operations of ships in international waters? And where is the oil going? The war in Ukraine has led to a major reshuffling of trade flows, as Asia becomes an ever bigger market for Russian crude. Presenter: Tim Whewell Producer: Monica Whitlock