Seven of the surviving metope sculptures on the Parthenon have been removed from the building & placed and replaced with casts, in order to protect the originals from harmful air pollution.
CBC News (Canada) 
Parthenon sculptures to be replaced with replicas
Last Updated: Friday, December 22, 2006 | 9:46 AM ET
Greek authorities will move seven sculptures from the Parthenon into a museum and replace them with replicas to avoid damage from acid rain.
Natural erosion and pollution are taking a toll on the marble metopes.
Metopes are sculptures carved into square blocks in the friezes that stretch in a band above the columns of the ancient structure.
The metopes, most of battle scenes, will be exhibited in the new Acropolis Museum due to open next year, said Maria Ioannidou, director of the Acropolis restoration services.
The metopes are among the Parthenon’s few remaining original sculptures, dating from around 447 B.C. They will be replaced by replicas to maintain the structure of the building.
The sculptures have also been defaced by marauders over the centuries, including damage done by Christians when the Parthenon was converted into a church.
Removal of the sculptures is to be completed by 2008, archeologists said. Greece is in the midst of a major project to restore and preserve the fragile buildings of the Acropolis.