The Elgin Marbles are some of the remaining fragments of sculpture that originally were part of the Parthenon. They have been in the British Museum for something like 200 years. For the the last 30 years, Greece has asked that they be sent back to Greece, but the Brits have refused for a variety of reasons. The dialogue has gotten really heated in more recent years, with some who side with the Greeks even accusing the Brits of stealing the marbles from Greece. I have very strong view on this debate, and I'm going to air them here because no one I know has the least interest in discussing this subject with me.
First of all, the British did not steal the Elgin Marbles. The Museum's ownership of them is completely legal. On top of that, anyone who cares about the marbles should thank the British for acquiring them, as it is highly unlikely they would have survived to this day without the careful preservation and safety in which they have been kept by the British Museum. So there is no justification to call the Brits thieves (at least not in this case). Further, even if the Brits did steal the marbles, Greece has no standing to demand their return. Britain acquired the marbles from the Ottomans (who controlled Greece at the time). I believe some feel that the Ottoman authorization shouldn't be valid since they had no vested interest in the antiquities of any country they were occupying. However, this position really just undermines the Greek position. The marbles belonged to the city state of Athens, so only the Athenian government would have any right to demand their return (as being the 'original owners'). But that government no longer exists.
So, the marbles belong to the Brits and they are justified to keep them as long as they like, especially since the Greeks have been so completely insulting about the whole thing. However...
Sometimes being 'right' doesn't mean you're doing the right thing. Despite the legality of the acquisition, the British Museum should return the marbles for a reason that transcends politics, nationalism, morals, and law. The marbles should go back to reunite - as much as can be done - a piece of art that should ideally never have been separated in the first place: the Parthenon.
The Parthenon is more than just a building; it is a piece of art that visually represents everything that ancient Athens stood for. It is a touchstone of heritage for the entire Western World. Now the British Museum often says that there's no point to return the marbles since they can never be reattached to the Parthenon. Maybe they can't be, but that's irrelevant. Seeing the marbles in the British Museum irretrievably divorces them them from their intended context. It would be like seeing the head of the 'David' statue - without the body - in a museum in Cleveland and expecting to get an feel for what the whole sculpture. It's impossible to get the real - and intended - impact.
I can say this confidently because I have been to the Parthenon. While I understand where the Elgin marbles 'fit' into the whole, it would have been far better to see them at the same time as I saw the rest of the building, even if they were not attached. At least then I have the best context possible for viewing this masterpiece of human architecture and art.
I do not think this would lead to a slippery slope requiring the return of all antiquities to their country of origin. It's not a matter of a single statue or a painting; this is a piece of art that has been broken apart and spread across the globe (for whatever reason). Reuniting the pieces makes this a fairly unique case.
For the Elgin Marbles and - by extension - any piece of the Parthenon, whether the piece belongs to Greece, Britain, or all humanity isn't the relevant issue. Nor is it relevant to discuss how long some part of the Parthenon has been somewhere in determining whether it should go back. The important question to ask is: Do we or do we not want to preserve great pieces of art? If the answer is yes, then the British acted to save an important part of this major site, and they should now complete this act by helping to reunite the pieces of the Parthenon as best as they are able. The Parthenon is a unique object, an architectural artwork, and keeping it scattered across the world - for any reason - is not in the best interests of anyone who cares about art or history.