The Great Sphinx of Giza is commonly believed to have been built during the reign of Pharaoh Kafrah (c. 2558 – 2532 BC). That makes it the oldest known monumental sculpture. It is also the largest monolith statuerecep. Made from limestone such a long time ago the couchant Sphinx was of course subject to natural erosion. But there is some merit to the claim that the natural process is accelerated by acid rain now, especially if you have seen how the Moloch of Cairo, the loudest city in the world, is closing in on the reclining lion with a human face.