Across Europe, and the world at large, there are a large number of archeological sites yet to be discovered. Many of these ancient settlements have disappeared from the face of the earth because their structures were made predominantly of wood and earth. Wood decays and any earthen feature has since been eroded by centuries of ploughing. Nevertheless, evidence of prehistoric activity survives below ground, and during spells of extreme drought and heat, these ghosts of the past appear mysteriously through patterns in crop growth. They are known as cropmarks.

Cropmarks at Grézac, France. Photo credit: cliché J. Dassié/Wikimedia