On satellite images parts of several "rings" are recognizable, similar in their proportion with the ring system by Plato. It is not known if any of these shapes are natural or manmade and archaeological excavations are planned. Geologists have shown that the Doñana National Park experienced intense erosion from 4000 BC until the 9th century AD, where it became a marine environment. For thousands of years until the Medieval Age, all that occupied the area of the modern Marshes Doñana was a gulf or inland sea-arm, but there was not even a small island with sufficient space to house a small village.