About 201–34 million years ago, from the Jurassic to the Eocene, Greece was continuously under the sea. The Tethys Sea was a large ocean that existed between two lithospheric plates (pieces of the Earth’s crust). On the seafloor of this sea, sediments accumulated, which today form the limestone rocks. Later, during the Oligocene to the Middle Miocene (34 to 13.82 million years ago), although Greece was still below sea level, the Tethys began to recede due to the collision of the lithospheric plates. This marked the beginning of the uplift of the Greek landmass and the formation of the first mountains. Material from these early mountains was carried into marine basins by rivers, creating new sediments atop the older ones, which today form the flysch.