Were deserts once oceans/seas?

Question

I was reading The Alchemist and stumbled upon something that says,

Here and there, he found a shell, and realized that the desert, in remote times, had been a sea.

Were the deserts once oceans/seas?

Answer 1

Yes, some of them clearly were, for example whale fossils have been found in the Sahara at the Wadi al-Hitan. However, while some deserts were once oceans/seas, that doesn't mean all deserts were once oceans.