On November 16, 1974 SETI researchers at the Arecibo radio telescope in Arecibo, Puerto Rico, beamed a message into space. They fired it toward Messier 13, a globular cluster in the Hercules constellation, which rests 25,000 light years away.
The message composed by Dr. Frank Drake, Carl Sagan, and other scientists, was conveyed using frequency modulated radio waves, converting it into a three minute binary transmission consisting of roughly 210 bytes.
The transmission produced an image and when decoded, the message provides the following information:
Numbers from 1 to 10
The atomic numbers of the elements that form deoxyribonucleic acid (DNA)
The formulas for the sugars and bases in the nucleotides of DNA
The number of nucleotides in DNA, and the double helix structure
A human figure, the physical dimensions of an average man, and the human population of Earth
A representation of the Solar System
A graphic of the Arecibo radio telescope and the dimensions of the transmitting antenna dish
The signal was a million times stronger than the typical TV transmission. 27 years later in 2001, we did receive a response!
On August 21, 2001 two crop circles popped up near the Chilbolton Observatory in Hampshire, UK.
One resembled a human face but the other has since dubbed the "Arecibo Answer," as it contains a format nearly identical to the original 1974 transmission, albeit with a few changes.
Crop circle: "Arecibo Answer"
The response, when decoded, leaves us with several differences.
Most interestingly, however, is another message encoded within a crop circle (known as the Crabwood formation) that appeared in 2002, also in Hampshire, UK. This particular crop circle contains the image of an extraterrestrial along with a circle, or disc, containing yet another binary message. The message decoded using 8-bit binary ASCII code.