Plunder or Preservation? Where do Artifacts of Historical / Cultural Significance Belong?

Museums around the world are in possession of thousands of pieces of art and artifacts which were created by people of a different culture, oftentimes in a different part of the globe.  Is there a moral obligation to return these objects to their place of origin?  What would this mean for our ability to preserve these irreplaceable items?  Do we lose the capacity to effectively educate the public if they are removed from museums?