The 'face on Mars' is one of the most intriguing and best-known images in planetary science.
It's located in the Cydonia region of the Red Planet, and since that first image was taken in 1976, there have been at least a few voices convinced it's evidence of intelligent life on Mars.
But subsequent, more recent images show the formation in a new light, revealing the structure's true nature.
So what actually is the 'face', why does it look like this and where did the image come from?
Here, we'll delve into the facts behind the photo.
The first face on Mars image
The original 'face on Mars' image was captured by NASA's Viking 1 Orbiter spacecraft on 25 July 1976.
Viking 1 and its successor, Viking 2, were both designed with the goal of reaching Mars and photographing its surface, gathering information that scientists could use to investigate the possibility of life on the Red Planet.
Many consider Viking 1's lander to have completed the first 'successful' landing on Mars, the caveat being that the Soviet Mars 3 lander, which did land on the planet in in 1971, lost contact with ground control about 2 minutes later.
At the time the Viking 1 orbiter took the image of the face on Mars, it was scouting a location for the future Viking 2 mission.
The two Viking spacecraft would together eventually capture a total of 52,663 images of Mars and map 97% of the planet's surface.
What's more, the orbiters' landers captured 4,500 photos of their respective landing sites.
You'll notice black speckles all over the original face of Mars image, and this, NASA says, is a result of missing data – 'bit errors' – caused by issues in transmitting the Viking 1 data back to Earth.
These bit errors make up part of one of the 'eyes' and 'nostrils' of the face on Mars.
The formation is actually an eroded rock formed from natural, geological processes on Mars, and stretches 1.5km (1 mile) across.
At the time the image was taken by Viking 1 from 1,873km (1,162 miles) away, the Sun was at an angle of about 20°, casting shadows that complete the 'face'.
A new look at the face on Mars
NASA got a chance to look again at the face on Mars in April 1998, when it was photographed by the Mars Orbiter Camera on the Mars Global Surveyor spacecraft.
The image was processed at the Malin Space Science Systems facility.
Mars Global Surveyor was a NASA spacecraft that orbited Mars and sent data back to Earth for almost exactly 10 years, from November 1996 until November 2006.
Its more recent view of the face on Mars, visible above, was captured just after the Mars Global Surveyor's 220th close approach to the Red Planet, on 5 April 1998.
At the time the image was taken, the face was 444km (275 miles away), the Sun about 25° above the horizon.
The image's resolution is 10 times higher than the Viking 1 image captured in the 1970s.
In the collage above, three images of the face on Mars can be seen.
The left image is a section of Viking 1's original image, which has been enlarged with modern image processing techniques.
The middle image is a portion of the Mars Global Surveyor image shown 'normally', while the right image is the same Mars Global Surveyor image, processed so that its brightness matches the approximate conditions of the Viking image.
In these clearer images, free of bit errors and other artefacts, the face on Mars disappears.
Then on 8 April 2001, the Mars Global Surveyor spacecraft once again captured an image of the face on Mars, which you can see above.
The image was captured while Mars Global Surveyor was looking at the feature from 165km (102 miles) to the side and from a distance of 450km (280 miles).
Again, this newer image, captured with more modern equipment, shows the face on Mars for what it really is: a geological formation that, when seen clearly, looks very little like a face.
Face on Mars in 3D
In 2006, the European Space Agency's Mars Express orbiter captured new data of the Cydonia region that enabled the construction of a 3D view of the face on Mars, which you can see above.
It is possible to view this 3D image and discern potential facial features.
But any truly unbiased view of the image is likely to conclude that it is indeed a random collection of geological features.
So what's going on?
Should it need pointing out, the 'face on Mars' isn't really a face, nor is it an alien structure made by an intelligent species.
While numerous Mars spacecraft and rovers have found evidence that Mars once had liquid water on its surface, and could potentially have the right chemicals to support life, to this date there is no concrete evidence even of microbial life on Mars, let alone a fully-fledged, intelligent species.
Like so many things in astronomy, from clair-obscur effects on the Moon to nebulae that look like animals, the face on Mars is an example of pareidolia, where the human brain sees faces or other familiar patterns in random objects.
The phenomenon is thought to be an evolutionary trait that once helped our ancestors to spot potential predators or other dangers from a distance.
A good example of this is the famous supernova remnant known as the Crab Nebula.
Modern images show it looks nothing like a crab.
But the name came from a sketch produced in 1844 by astronomer Lord Rosse, using the famous Leviathan telescope at Birr Castle in Ireland.
With limited observational capability, it looks like a crab (according to Rosse's sketch), but with modern telescopes, it's clear it looks nothing like a crab.
The same is true of the face on Mars. Newer, clearer images reveal it doesn't look like a face after all
That hasn't stopped the face on Mars from gaining traction in conspiracy theory circles over the decades, however.
In that original 1976 image, it really does look like a face, or perhaps even – if we wanted to run with the idea for a moment – a statue or monument built by an intelligent Martian species.
But why, if there is indeed a an alien species living on Mars with the engineering prowess to construct a such a monument stretching 1.5km across, but there's yet no evidence of buildings, transport systems or other infrastructure, is anyone's guess.
