Not as difficult as one might think. Turns out there is an immense amount of water locked up underground in mineral deposits “three times greater than all of Earth's surface oceans combined.”

Unusual tectonic activity may have caused it to release and reabsorb.

Also remote glacial shifts could explain periods of immense flooding. And then there is the Euphrates.

Or tsunamis.

Lots of evidence of immense floods throughout history.

Apparently the sphinx shows signs that it sat in a rainy jungle once upon a time. Over a long enough time line no one can really say what’s been going on.