Back in the 1990s a raging controversy erupted around the geology of the Great Sphinx, specifically regarding the appearance of water worn surfaces on the body of the Great Sphinx and on the walls of its surrounding enclosure. Robert Schoch and John Anthony West argued that this weathering was produced by heavy rainfall, which could only have occurred at a time prior to the 4th dynasty of Egypt around 2500BCE, meaning the Great Sphinx was older than archaeologists had traditionally assumed.

A 1993 documentary, The Mystery of the Sphinx, outlining the controversial view can be see on Youtube (divided into 9 ten-minute parts) at this link.

The Hall of Maat maintains a page with several of the articles that were exchanged back and forth during this academic bun fight. The articles can be found at this link.

More recently, Robert and Olivia Temple argued that the Great Sphinx had at one time been surrounded by a moat, thus providing an alternative explanation for the seemingly water eroded surfaces on the Sphinx and it's surrounding walls. The webpage relating to their book can be found here.

Robert Schoch summarises his theories, as well as countering the hypothesis of Robert and Olivia Temple, at this page here on his offical website.