The Parthenon Marbles were not stolen but were given by an Ottoman Sultan to Lord Elgin, claims a descendant of the man who took them to Britain.

Lord Charles Bruce said the marble friezes, now on display in the British Museum, were given by a Turkish sultan to his forebear Thomas Bruce, the seventh Earl of Elgin. In return, the Earl gave him a chandelier for his palace — as well as the vaccine for smallpox.

The marbles were chipped off from the ruins of the Parthenon Temple in Athens and shipped to England between 1801 and 1805. At the time, Greece was part of the Turkish Ottoman Empire.
In a recent interview, Lord Bruce told The Times that...

"The marbles were a diplomatic gift. It’s a part of the story not clearly understood. The British had cemented a military alliance with the Turks, and there was a personal friendship between Elgin and the sultan. They exchanged gifts, and there’s a beautiful chandelier from Elgin which still hangs in the Topkapi Palace [in Istanbul]. The chandelier still hangs in a room where Lady Elgin taught the sultan’s family to dance the eightsome reel. We also gave them the smallpox vaccine, which prevented an outbreak in Smyrna, and later the vaccine went on to Baghdad and Bombay, and was used to inoculate a million Indians."

The Parthenon Marbles, also known as the Elgin Marbles, is a collection of classical Greek marble sculptures, inscriptions and architectural members that originally were part of the Parthenon and other buildings on the Acropolis of Athens. It is widely believed that Thomas Bruce, the 7th Earl of Elgin obtained a controversial permit from the Ottoman authorities to remove pieces from the Parthenon while serving as the British ambassador to the Ottoman Empire from 1799 to 1803. Since then, there has been great controversy surrounding the legitimacy of this permit and the validity of the UK's claim to keep the Marbles instead of sending them home to Greece. The UK, however, maintains its claim.