In the middle of the sand-covered northwest of Africa, in the heart of Mauritania, is one of the most recognizable geological features on Earth – the Richat Structure, which astronauts affectionately call the "Eye of the Sahara" or the "Eye of Africa". This almost perfect circle in the desert is so large that it is easier to understand from space than from the surface itself. Although it once sparked the imagination as a possible trace of a giant-meteor impact, today we know that it is an extremely eroded geological dome shaped by the Earth's interior itself.

Where is the "Eye of the Sahara" located and how big is it?

The Richat Structure lies on the edge of the large Taoudeni Basin, on the Adrar Plateau in the northern part of Mauritania, not far from the historic city of Ouadane. It is located in the middle of the Sahara, in the Adrar region, which is characterized by high stone plates, deep erosion valleys, and long sand dunes. The diameter of the structure is estimated to be between approximately 40 and 50 kilometers, depending on whether only the most visible annular part is counted or also the wider geological zones belonging to it. It is precisely this size that explains why the "Eye of the Sahara" has become a favorite landmark for astronauts on the International Space Station and the crews of numerous missions since the first crewed flights.

When the images are observed from orbit, concentric rings of lighter and darker shades are clearly visible, resembling a huge target or a spiral eye. The uniform color of the desert dominates the background, which further emphasizes the circular pattern of Richat – almost as if someone drew a regular geometric sign in the center of the Sahara.

From "meteorite crater" to geological dome

The first scientific interpretations of the Richat Structure in the mid-20th century were marked by uncertainty. Due to its raised rim, lowered center, and circular shape, it seemed logical that it could be the trace of a large meteorite impact. This interpretation was further encouraged by the fact that other, confirmed meteorite structures are located in the wider area. However, detailed field research, laboratory analyses, and later satellite studies showed that key impact traces were missing: there is no shocked quartz, no glassy melts, nor other typical "visiting cards" of a cosmic collision.

Instead, geologists today describe Richat as a large, slightly elliptical dome – a structure formed deep within the Earth's interior when a magmatic body pierced and uplifted older sedimentary layers. This process of intrusion and uplift deformed the rock layers, and later millions of years of erosion by wind, water, and sand gradually stripped away the upper parts of the dome, revealing concentric rings of differently resistant material. The oldest rocks are found today in the center, while younger formations ring the core.

The age of the Richat Structure is tied to the Cretaceous period – geologists estimate that the underlying magmatic complex is about 100 million years old. During that long period, alternating periods of weathering, rainier phases, and extreme aridity further emphasized the relief differences between the tougher and softer rock layers.

Rings of quartzitic sandstone and softer rocks

At first glance, images of Richat look almost abstract: the rings alternate from dark to light tones, and individual segments look as if they belong to a completely different landscape. Geological analysis reveals that this is a comparison of very resistant and much weaker rocks. The most pronounced annular barriers are formed by resistant quartzitic sandstones, which stand out as high ridges or "ramparts" around the center. Between these more resistant rings are softer layers of limestone, mudstone, and other sedimentary rocks that erode more easily, thus forming lower, darker valleys.

Satellite images in the visible spectrum show quartzites as shades of light brown to reddish color, while the eroded valleys are darker. In false colors – for example, in combinations of the near-infrared part of the spectrum – the contrast is even more pronounced: resistant rocks often stand out in shades of red and pink, while depressions take on darker green, purple, or brown tones. According to geological measurements, the central rings are uplifted about 80 meters above the lowest parts of the structure, which is not a dramatic difference in height, but it is enough to create clearly expressed step-like shapes that satellites record from orbit.

In the layers exposed on the surface within Richat, an entire mosaic of rocks has been found: from Late Proterozoic to Paleozoic sediments, through eruptive rocks such as rhyolite and gabbro, all the way to specific carbonatites and kimberlites. All this confirms that it is a deeply rooted, complex magmatic system in which hydrothermal fluids and heat from the interior played a key role in rearranging the rock composition.

Adrar Plateau and Erg Ouarane – the stage around the "eye"

The Richat Structure does not exist in a vacuum, but is part of a broader geological and landscape context. The darker belt seen around the rings in many images is part of the Adrar Plateau – an uplifted slab of sedimentary rocks that rises about 200 meters above the surrounding sandy areas. The edges of the plateau look like inverted cliffs that drop sharply towards the vast dunes, making it seem as if the "Eye of the Sahara" lies on the border of two worlds: the stony highlands and the sea of sand.

Southeast and south of the structure stretches Erg Ouarane, a massive sandy desert that extends for hundreds of kilometers towards Mali. Satellite images clearly show how the dunes are slowly "entering" the Richat circle, especially on its southern side. The sand, like yellow gas, creeps into the hollows between the rings, covering parts of the once-exposed rocks. This is proof that this landscape is not static even today: the winds of the Sahara continue to tirelessly move material, changing the appearance of the structure on time scales that are slow from a human perspective, but surprisingly fast from a geological perspective.

Despite the extremely arid climate, traces of water and former river flow are still visible. Dry riverbeds, which are barely discernible on natural images as shallow furrows, take on clearly highlighted lines in false colors. Along these dry valleys, where groundwater occasionally comes closer to the surface, rare trees and shrubs grow. In high-resolution satellite photographs, this vegetation appears as tiny dark dots, and in the infrared spectrum as purple or dark red patches – a discreet but persistent reminder of life in an almost completely inhospitable environment.

How Sentinel-2 "sees" the Richat Structure

The Richat has been photographed from space so many times in recent decades that it has become a kind of icon of Earth observation. Particularly impressive images come from the Copernicus Sentinel-2 mission, part of the European planet observation program. The two identical Sentinel-2 satellites fly in a polar orbit and carry a multispectral instrument with 13 channels, from the visible part of the spectrum to the short-wave infrared range. Thanks to a spatial resolution of up to 10 meters, it is possible to monitor not only large geological entities but also finer details of relief, vegetation, and surface changes.

Special insight into the "Eye of the Sahara" was provided by Sentinel-2 satellite images taken at the end of September 2025. In natural colors, Richat is shown as the human eye would see it: the rings alternate in shades of brown, beige, and grayish color, while the surrounding dunes have a more uniform yellow-orange color. But when the same scenes are displayed in false colors, a combination of infrared and visible channels, details emerge that otherwise remain hidden – the resistant quartzitic rings "light up" in various shades of red and pink, while softer rocks and sandy surfaces take on darker tones.

Such color combinations are not just visually attractive; they help scientists distinguish individual rock types, track the degree of erosion, notice changes in soil moisture, or the presence of rare vegetation. In the case of Richat, false colors clearly emphasize the annular texture and reveal where erosion is occurring faster, and where the rocks are still resistant enough to form almost closed rings.

Why is the structure easier to see from space than from the ground?

On the ground, within Richat itself, an observer would find themselves among hills and valleys that at first glance do not reveal that they are part of a regularly organized system of rings. The local relief consists of a series of low ridges, rocky slopes, and sandy hollows, and the differences in height often do not exceed a few tens of meters. Without a wider perspective, it is difficult to even guess that these ridges are parts of a huge concentric shape.

That is precisely why satellite missions and images from high altitudes play a key role in understanding this structure. The combination of data from the visible, infrared, and short-wave infrared parts of the spectrum, along with radar and gravitational measurements, allowed geologists to reconstruct the internal structure of the dome, the directions of the layer dips, and the positions of annular faults in the deeper subsurface. Based on such data, detailed geological maps were created that explain how the magmatic body uplifted the sedimentary layers, where cracks formed, and how hydrothermal fluids moved through the rocks over time.

Age and depth of the geological history of the "Eye of Africa"

The Richat Structure offers a rare glimpse into the deep history of the Western Sahara. The central part reveals some of the oldest rocks in the region, dating from the late Proterozoic, while the peripheral rings represent younger, Paleozoic sediments. This geological "footage" allows scientists to track changes in sedimentation, climatic conditions, and tectonic processes through hundreds of millions of years in a relatively small area.

The magmatic activity that caused the uplift of the dome is linked to the movements of tectonic plates and changes in the internal dynamism of the Earth during the Cretaceous period. The intrusion of magma into existing sedimentary layers created annular faults and fissures through which hydrothermal fluids later circulated, dissolving one part of the minerals and depositing new ones. This is how the characteristic siliceous breccia in the center of the structure was formed – a rock composed of crushed and re-cemented fragments of different lithologies.

Although today we do not have a completely unified model that explains all the details of Richat's formation, most research agrees on the key elements: it is a structure formed by a combination of magmatic uplift, annular faults, and long-term differential erosion. This makes it one of the most interesting natural laboratories for studying domes and annular structures on Earth and, indirectly, potential similar forms on other planets.

Richat between science and popular myths

As is often the case with striking natural phenomena, the Richat Structure has also sparked a series of speculations outside of strictly scientific frameworks. In recent years, various theories have circulated on the internet linking this area to the legendary city of Atlantis or other lost civilizations. The arguments are mainly based on the visual similarity of the concentric rings to some ancient descriptions, while geological data clearly indicate that it is a naturally formed structure whose age and genesis have nothing to do with human history.

Nevertheless, popular myths somewhat contribute to the global recognition of the "Eye of the Sahara," attracting public attention and stimulating interest in scientific explanations. Geologists point out that, although there is no evidence of the artificial origin of the structure, its complexity and conspicuousness make it an ideal example of how natural processes can play with our expectations and create shapes that seem almost "designed."

Human presence and access to the hard-to-reach landscape

The Richat Structure is located in a relatively sparsely populated part of Mauritania, where nomadic communities and small towns dominate. The nearest larger historical center is Ouadane, a city with a rich caravan past, which is also inscribed on the UNESCO World Heritage List due to its historical core and importance on the former trans-Saharan trade routes. Although Richat itself is not yet part of the official World Heritage List, it is often mentioned as a potential candidate due to its unique geological and scientific value.

Access to the structure itself requires careful planning: the distance from larger urban centers, extreme temperatures, water scarcity, and demanding terrain conditions mean that visits are most often organized as part of specialized expeditions or tourist tours with local guides. Those who do decide to make the journey are rewarded with the opportunity to walk the rings that represent a cross-section through hundreds of millions of years of geological history, and to closely observe the transitions between the mountains of quartzitic ridges and the sandy valleys.

What does the "Eye of the Sahara" tell us about the future of deserts?

In addition to being a natural laboratory for studying geological domes, Richat is also an important case for understanding the dynamics of desert landscapes. Satellite missions like Sentinel-2 enable a long time series of observations, making it possible to track how sand dunes slowly advance, how the tones of the rocks change due to weathering, and how rare vegetation reacts to occasional precipitation episodes.

In the broader context of climate change, such places offer insight into the processes of desertification, sand movement, and changes in water availability. Although Richat is primarily a geological formation, the fact that it is located at the transition between a rocky plateau and mobile dunes makes it a sensitive indicator of changes in the wind and precipitation regime. Comparing images over several decades can reveal the expansion or contraction of sand fields, as well as possible changes in the surface runoff of rare flash floods.

Sentinel-2 and future research of the Richat Structure

The Copernicus Sentinel-2 mission was conceived primarily as a tool for monitoring land surfaces, vegetation, agriculture, and natural disasters, but its high spatial and spectral resolution has proven to be extremely valuable for geology as well. In the case of Richat, the continuous collection of data allows for the monitoring of erosion, potential landslides, the position of sand dunes, and changes in the reflection properties of the rocks.

As the Copernicus data archives fill up with an increasing number of images, researchers get the opportunity to compare scenes recorded in different seasons and years. This can help in finer modeling of how sediments move within the structure, how the colors and textures of the rocks change, and what the dynamics of rare vegetation are in dry valleys. In combination with other satellite missions, radar, gravimetric measurements, and field research, Richat will likely remain a reference point for studying annular geological formations for a long time to come.

For European and global Earth observation programs, the "Eye of the Sahara" represents an ideal visual motif that clearly demonstrates to the public how data from space can reveal the planet's hidden structure. For geologists, it is a complex enigma whose solution has been slowly pieced together for decades. And for the satellites themselves that fly over Mauritania every day, Richat has become an indispensable "check point" – a formation whose rings will continue to attract attention every time a new image appears in the archives.