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.

The water is the normal ocean water.

Plate tectonics, fossilized remains of fish/animals in the desert, and an uneducated population demanding answers from the priest/leadership class explains the rest.

They just made it up. For control.

"God told [the elites], you must obey [the elites], or else."

Evidence: We still tell our children outrageously fraudulent stories with embedded morals in order to alter their behavior.

It's the most likely explanation we have.