Marble is damaged when exposed to excessive rain, especially if the rain water contains lots of acid. E.g. At the Parthenons of Athens, Greece, black crust and coatings were present at the marble surface.
Many a times to protect a marble structure that is being harmed by such natural phenomenons, they are relocated. But many people oppose this on moral and artistic grounds. Due to this many of the original marbles have been returned to their original cities. E.g. The Getty Museum in Los Angeles and The University of Heidelberg, Germany have returned the marbles back to Athens.
Hope people take proper steps to protect their marbles and also marbles of ancient value return back to its original place.