The Arecibo answer is a crop circle — or crop rectangle — that is purported to be a response to the Arecibo message, a piece of coded information about Earth and humanity beamed into space in 1974. It appeared in 2001 near the Chilbolton radio telescope in Hampshire, UK. Despite crop circles being known to be hoaxes, some believe this is key evidence of extraterrestrial presence on Earth.
The crop pattern is a near-replica of the Arecibo message, which contained various pieces of information such as the numbers of chemical elements, the composition of DNA, the position of Earth in the Solar System and a depiction of a human being. The “answer” itself doesn’t expand much upon this and still forms the same 23×73 grid (because these numbers are primes, so a recipient could guess that a 1679-bit message should be arranged in a rectangle) and most of the chemical data remains the same.
The changes in the response are straight from existing alien folklore and science fiction. In the section detailing important chemical elements, the main focus is altered from carbon to silicon, and the diagram of DNA is re-scribbled slightly. At the bottom, the pictogram of a human is replaced with a shorter figure with a large, bulbous head: a grey alien, which as a depiction can only be something that a human would come up with. In blatantly trolling the UFO conspiracists, there is a nearby depiction of Richard Hoagland‘s alleged face on Mars.
The likelihood of the Arecibo message ever being picked up is very, very low. It was aimed at globular star cluster M13, which is 25,000 light years away. In fact, it’s so far away that in 25,000 years, its destination will in fact have moved! The message was only intended as a demonstration of the transmission technology, not as a serious attempt to make contact.
The message will also pass close enough to a few nearby stars to potentially be “received” in their vicinity. But why wouldn’t the recipients simply send back a message via radio, instead of coming here and messing up some poor farmer’s crops at night and then sneaking away?