The Sphinx is built/cut out of limestone. The limestone on which it sits is like, limestone everywhere, full of cracks and fissures from water
movement
In 1973-4 . . . the first in a series of serious pioneering projects was launched, using ground-penetrating radar and other high-tech remote
sensing equipment to locate "anomalies" under the bedrock beneath the Sphinx. These projects were channeled through well-established academic
institutions - the Ain Shams University in Cairo and the prestigious Stanford Research Institute (SRI) in the USA.
There was a another search in 1977
In the first one cameras were sent down....and they found normal limestone fissures.
No one is going to excavate under the Sphinx because to do so your going to have to cut thru previously UNCUT limestone, that isn't excavation that
mining.