Cydonia is a region in the northern hemisphere of Mars, on the boundary between the cratered southern highlands and the smooth northern plains, where Arabia Terra meets Acidalia Planitia. Named after an albedo feature that borrows the name of the ancient Cretan city of Kydonia, it holds the flat-topped mesas of Cydonia Mensae, the hills of Cydonia Colles and the valleys of Cydonia Labyrinthus. Its fame outside planetary science comes from one of those eroded hills.

On July 25, 1976, the Viking 1 orbiter, scouting a landing site for Viking 2, captured a mesa about 1.5 kilometres across that looked like a human face. NASA said the formation resembled a human head, and the image caption explained that shadows gave "the illusion of a nose and mouth"; Viking chief scientist Gerald Soffen called it a trick of light and shadow. The NASA Goddard computer engineers Vincent DiPietro and Gregory Molenaar later found a second Viking frame of the Face on Mars, taken 35 orbits afterward under a different sun angle.

The best-known advocate of an artificial origin is Richard Hoagland. In The Monuments of Mars: A City on the Edge of Forever (1987) he argued that the Face was part of a city of pyramids and mounds left by an ancient civilization, and he has accused NASA of concealing the evidence. The mathematician Ralph Greenberg wrote that the logic of Hoagland's geometric deductions was flawed, Carl Sagan criticized such speculation in The Demon-Haunted World (1995), and Hoagland received a 1997 Ig Nobel Prize "for identifying artificial features on the moon and on Mars."

Sharper images answered the question for planetary scientists. Mars Global Surveyor photographed the mesa in 1998 and 2001, ESA's Mars Express imaged Cydonia at about 13.7 metres per pixel in July 2006, and NASA's Mars Reconnaissance Orbiter captured it on April 5, 2007 at about 30 centimetres per pixel, showing a weathered butte; ESA summarized the scientific view of the Face as "a figment of human imagination in a heavily eroded surface." The claims continue in 2026: an American Alchemy episode with the author Timothy Alberino revisited the Viking images and the idea of Martian ruins, and the physicist Max Derakhshani promoted the independent researcher Holger Isenberg's report that China's Tianwen-1 orbiter imaged Cydonia in 2021 under low light similar to Viking's, which Derakhshani argued makes the Face look more artificial. Cydonia remains planetary imaging's best-known case of a landform first seen at about 50 metres per pixel and later resolved into an eroded hill.