Crop circles were shapes made from flattened crops. They were generally thought to be a sign of alien activities.
Maxwell Edison was investigating a crop circle in Stockbridge before being caught up in an adventure with the Eighth Doctor. (COMIC: Endgame)
A crop circle was seen in Kent. It was noticeable for being made of square-sided shapes and was filled with ice. (PROSE: The Algebra of Ice)
Donna Noble investigated crop circles in the hopes of finding the Tenth Doctor. (TV: Partners in Crime)
In 2011, Amy Pond and Rory Williams made a crop circle with their car to attract the attention of the Eleventh Doctor. (TV: Let's Kill Hitler)