Arecibo message

The Arecibo message was a 1974 interstellar radio message carrying basic information about humanity and Earth sent to globular star cluster M13 in the hope that extraterrestrial intelligence might receive and decipher it.
The Arecibo answer is a crop circle (well, crop rectangle, to be accurate) that is purported to be a response to the "Arecibo message", a piece of coded information about Earth and humanity which was first beamed into space in 1974.