The Great Sphinx of Giza, a giant limestone figure with the body of a lion and the head of a man wearing a pharaoh’s headdress, is the national symbol of Egypt—both ancient and modern—and one of the world’s most famous monuments. Despite its iconic status, geologists, archaeologists, Egyptologists and others continue to debate the Sphinx’s enduring “riddle”: Exactly how old is it? The most common wisdom holds that the monolith is around 4,500 years old, and was built for Khafre, a pharaoh of Egypt’s Fourth Dynasty who lived circa 2603-2578 B.C. His pyramid is the second tallest of the pyramids built at Giza, next to his father Khufu’s Great Pyramid. To make up for its lesser size, Khafre’s pyramid was built at a higher elevation and surrounded by a more elaborate complex with numerous statues, including the Sphinx, the head of which is thought to be built in the pharaoh’s own image.

Not everyone believes that the Sphinx was built for Khafre, however. As far back as the mid-19th century, some Egyptologists pointed out that even though the Sphinx is located within the pyramid complex traditionally identified as Khafre’s, no contemporary inscriptions directly link him with the statue. Over the years, various researchers have credited the Sphinx to Khafre’s father, Khufu, and to Djedefre, another of Khufu’s sons. More recently, a new theory emerged that places the statue’s origins much further back, to some 9,000 years ago. Supporters of this hypothesis point to extensive erosion of the limestone near the top of the Great Sphinx, arguing that the last time the region experienced enough rainfall in the region to cause this type of erosion on limestone was 7000 B.C.

Dating the Sphinx back this far suggests the statue was the work of an advanced civilization predating the ancient Egyptians—an intriguing, if highly controversial, proposition. Most scholars still accept the traditional dating of the Sphinx to Khafre’s era, arguing that the new theory doesn’t take into account all the evidence on the table. Carved from the natural limestone of the Giza Plateau, known as the Mokkatam Formation, the Sphinx is known to erode very quickly, which would explain why it looks older than its age. Moreover, water drainage beneath the ground’s surface or flooding from the Nile River could have caused the erosion in question, rather than precipitation. According to the Ancient Egypt Research Association (AERA), the architectural and geological evidence both support the conclusion that the Sphinx and its adjoining temple were built along with the rest of Khafre’s pyramid complex, and were in fact among the last of the monuments to be completed.