The Eye of the Sahara is a giant bullseye in the Mauritanian desert, up to fifty kilometres across, and astronauts were already photographing it from orbit while geologists still believed it was a meteorite impact crater, long before anyone worked out it was something else entirely.

The Richat Structure, the concentric ring formation in the Adrar plateau of Mauritania often called the Eye of the Sahara, was photographed from orbit by the crew of Gemini IV in June 1965, at a time when the leading hypothesis on the ground was that it had been formed by a meteor impact. The orbital photograph reached the public before the geological consensus did.

The structure is about 40 kilometres across, depending on whether you count the outer rings, with some descriptions stretching that to 50. It sits near the town of Ouadane in northern Mauritania, on a plateau dry and bare enough that the rings of differing rock types stand out against the surrounding desert with the kind of clarity that mostly only works from high altitude.

It does not work from ground level. A traveller walking through the Richat Structure would see ridges, slopes, and gullies, and would not necessarily recognise that the ground beneath them is part of a near-perfect circle the size of a medium European city.

What the Gemini IV crew actually saw

James McDivitt and Ed White launched aboard Gemini IV from Cape Kennedy on 3 June 1965. The mission ran four days. White’s twenty-minute spacewalk on the second orbit became the photograph history remembered. The crew also took a large number of Earth-observation images as the spacecraft passed over Africa, the Middle East, and the Americas, sometimes opportunistically, sometimes on the flight plan.

One of those images, taken on 4 June 1965, captured the Richat Structure from orbit, showing the whole formation at a scale that helped turn it into a global curiosity. NASA’s Earth Observatory notes that the image helped bring wider global attention to a feature French geographers had already named and partly described from aerial photographs in the 1930s and 1940s, but which had not been seen at this kind of scale before.

This is the part of the story that gets compressed in popular retellings. The Eye was not discovered from space. It had been on French colonial maps for decades. What space did was give the world a single image of the whole thing at once, in a way that no aerial survey at lower altitude had managed.

The impact crater hypothesis and why it held

For most of the period between the structure’s first scientific description in 1948 by the French geographer Jacques Richard-Molard and the studies of the 1960s and 1970s, two ideas competed. One was that the Richat was a deeply eroded dome of uplifted rock. The other was that it was an astrobleme, the geological signature of an ancient meteor impact.

The impact theory had real reasons behind it. The Richat looks like a crater. It has a raised outer perimeter and a sunken centre. A 1952 expedition led by the French naturalist Théodore Monod found three nearby smaller features that were genuine impact craters: Aouelloul, Temimichat-Ghallaman, and Tenoumer. If the Sahara had been hit, repeatedly, by smaller bodies, the case for a larger neighbour seemed reasonable.

What undid the impact hypothesis was the absence of the things impacts leave behind. There was no central peak. There was no clear evidence of shock metamorphism: no melted rock, no shocked quartz with the diagnostic deformation lamellae produced by hypervelocity impacts. Early reports of coesite, a mineral that forms under impact pressures, were later identified as misidentifications. The pattern of features pointed somewhere else.

The somewhere else was an uplifted geological dome shaped by igneous intrusions, hydrothermal alteration, collapse, and long erosion, rather than a single neat crater-making event. Different rock types erode at different rates. The harder igneous rocks form the high ridges. The softer sedimentary rocks form the lower troughs between them. The bullseye is differential erosion, not impact geometry.

What the recent work shows

The hypothesis has been refined considerably in the last two decades. A 2005 paper by Guillaume Matton and colleagues in Geology, titled “Resolving the Richat enigma,” proposed a model in which doming above an alkaline igneous complex was followed by hydrothermal karstification, with the centre of the structure dissolved and collapsed by hot fluids working through the carbonate rock. A 2014 paper by Matton and Michel Jébrak filled in the alkaline-hydrothermal complex picture in more detail.

More recent work by Abdeina and colleagues, published in 2024 in Lithos under the title “How old is the Eye of Africa?”, attempts to date the igneous phases of the Richat more precisely. The picture that has emerged is of an isolated Cretaceous alkaline complex with a multi-phase history of intrusion, doming, and erosion. The International Commission on Geoheritage recognised the Richat Structure as a site of global geoheritage significance in 2022.

The impact hypothesis has not been retained by anyone working on the structure for a long time.

What the story is really about

The interesting thing about the Eye of the Sahara, in our reading, is not the structure itself, which is well understood by the people who study it. It is the lag between what could be seen from orbit and what was understood on the ground.

Earth-observation imagery from crewed and uncrewed orbital platforms has, since the 1960s, produced a steady stream of features that look like one thing from above and turn out to be another when investigated at the surface. The Richat is the famous example. There are others. A circular feature in the desert is not, by itself, a crater. A bright spot on a coastline is not, by itself, an algal bloom. The image suggests a hypothesis. The hypothesis is then tested by people with hammers, drill cores, and a willingness to be wrong.

That is the working relationship between orbital remote sensing and field geology, and it is not new. The Gemini IV photograph of the Eye of the Sahara is one of the first popular illustrations of the pattern. It is also a reminder that an image, by itself, does not constitute an explanation. It constitutes a question, well framed, from an unusually good vantage point.

The latest high-resolution images of the Richat, captured by Landsat 8 and 9 in March 2026, appeared as NASA Earth Observatory’s Image of the Day on 16 April. The structure looked the same as it had from Gemini IV. What had changed, in the intervening sixty-one years, was the explanation underneath it.