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.
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.