Aloba is one of those things you can't take in until someone walks underneath it. A sandstone bridge one hundred and twenty metres high with a span of seventy-six, hanging over a dry wadi in the eastern corner of the Ennedi, with nothing around it to help you measure it. Only when a human figure steps into the frame, a tiny dot at the base of a pillar twenty-three metres wide, does your brain accept the scale.
In the world rankings of natural arches, Aloba holds an odd position. It has the eighth-longest known span on the planet and is the largest outside China and Utah, but what really sets it apart is its height, far above the famous Rainbow Bridge in the US. And unlike the arches of Utah, there's no car park here, no trail and no interpretive panel: you have to cross the Sahara to see it.
The arch is the most spectacular piece of a massif made entirely of this stuff. The Ennedi shares a geology of horizontal sandstone with the Colorado Plateau, producing arches, towers, stone mushrooms and canyons on an industrial scale, and the scientific teams working there estimate that about 75 % of the massif has yet to be explored in detail. In other words, there are probably more Alobas out there that nobody has measured yet.
Aloba Arch is the product of tens of millions of years of erosion on a stack of sandstones laid down between the Cambrian-Ordovician and the Carboniferous on a granite basement. Water cut the canyons along the massif's fractures, and sand-laden wind attacked the softer layers, undermining the base of a rock wall until it broke through and left only the upper lintel standing: a stone bridge 120 metres high with a span of about 76, on pillars 23 metres wide. It has never been measured on the ground with surveying instruments. The figures in circulation come from photographic estimates documented by the Natural Arch and Bridge Society, given how remote the site is and how late it gained international recognition. In 2016 the arch became part of the World Heritage property «Ennedi Massif: Natural and Cultural Landscape», inscribed under cultural criterion (iii) and natural criteria (vii) and (ix), with 2,441,200 hectares of protected area.
Read the full history ↓A 60.000 km² sandstone massif full of arches, towers and canyons, with paintings 7.000 years old, crocodiles that outlived the green Sahara and eighteen lakes that shouldn't exist.
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Origins, formation and what the sources say.
The story of Aloba Arch begins at the bottom of a sea. The sandstones that make up the Ennedi massif were laid down between the Cambrian-Ordovician and the Carboniferous —roughly between 500 and 300 million years ago— on top of a much older granite basement, in horizontal layers of varying grain size and cementation.
That variability is the raw material for everything you see today. Some strata are hard, well cemented and resistant to abrasion; others are soft and crumbly, and give way easily to water and windblown sand. When the whole mass was exposed and began to erode, erosion did not act evenly: it attacked the soft layers first and spared the hard ones, and from that difference came the full repertoire of Ennedi landforms.
The comparison geologists keep making is with the Colorado Plateau, in the southwestern United States: the same recipe of horizontal sandstones, the same alternation of hard and soft layers, the same arid climate with occasional torrential rain, and therefore the same landforms —canyons, mesas, towers, stone mushrooms and natural arches—. The difference is one of human scale, not geological scale: where Utah has millions of visitors and paved roads, the Ennedi has four days of dirt track and a few hundred people a year.
The massif covers some 60,000 km², an area roughly the size of Switzerland, and peaks at around 1,450 metres. It gets between 50 and 150 mm of rain a year: almost nothing, but enough for every storm to strip away material, widen fractures and keep on sculpting.
A natural arch is not built: it is revealed by subtraction. The process has fairly recognisable stages, and all of them are on display in the Ennedi, which turns the massif into an open-air geological catalogue.
First, fracturing. The rock cracks along vertical planes, and water seeps into those joints and gradually widens them, carrying material away. Over a very long time, what was a continuous mass of sandstone ends up divided into partitions or fins: thin, isolated walls separated by sandy corridors.
Second, undercutting. At the base of those walls there is almost always a softer layer. Sand-laden wind and the water that collects at the foot attack that lower band first, carving out hollows and alcoves that eat into the wall from below. The hard layer above, meanwhile, holds out.
Third, piercing. When two hollows carved into opposite faces of the same wall meet, a hole opens up. From then on the process speeds up: the gap channels wind and water, widens, and thins the pillars, until what is left standing is a lintel resting on two legs. That is a natural arch.
And fourth, collapse. Every arch is a temporary structure. The lintel keeps thinning until it can no longer bear its own weight and falls. On the geological timescale, an arch is an instant. What we see at Aloba is a sandstone wall caught halfway between being solid rock and being rubble: a bridge 120 metres high with a span of about 76, held up by pillars some 23 metres wide, and a good deal thinner at the lintel than intuition would expect.
In the world of natural arches there is a specialist community that catalogues, measures and ranks these formations, with the Natural Arch and Bridge Society as its main reference. The standard measure for ranking them is the span: the clear distance between the supports.
At around 76-77 metres, Aloba ranks as the eighth-longest known natural arch on the planet and the largest outside China and the state of Utah. First place goes to Xianren Bridge, in China, with a span of about 122 metres. But ranking by span doesn't do it justice: in height, Aloba's 120 metres place it among the tallest arches known, comfortably beating the celebrated Rainbow Bridge in the United States, which is far more famous and far, far more visited.
One detail says a lot about the place: those figures don't come from a survey on the ground. They are estimates derived from photographs, precisely because the site is so remote and was recognised internationally so late. In other words, one of the largest natural arches in the world has still never been measured with instruments by anyone.
That fits the general state of knowledge about the massif. The scientific teams that have worked in the Ennedi for years estimate that around 75 % of the territory has yet to be explored in detail. In a 60,000 km² massif with the geology of the Colorado Plateau and a very high density of arches, the conclusion is almost inevitable: it's reasonable to think there are formations comparable to Aloba still waiting to be catalogued.
Aloba doesn't stand in a void. The Ennedi is home to nomadic and semi-nomadic herders —Toubou and Zaghawa, among others— who move through the massif with herds of camels, donkeys, sheep and goats, following the availability of water and grazing. Some 30,000 people are estimated to pass through the reserve each year.
That human presence is ancient. The massif's walls hold one of the largest collections of rock art in the Sahara, with dates going back 8,000 years: more than 86 % paintings and around 14 % engravings, with domestic animals making up about 69 % of the figures and cattle as the undisputed stars. It is the record of a green Sahara, of savannah and herds, made by the people who lived in it.
The sequence of animals depicted works almost like a timeline: first the cattle of the wet period, then horses —at Terkei there is a veritable army of riders painted on the walls— and finally camels, as in the extraordinarily detailed scenes at Manda Guéli, in the Archei area, where you can make out the African saddles, the swords, the ornaments and the riders' hairstyles.
In that landscape, the great rock formations serve an obvious practical purpose: they are landmarks in a territory without roads, reference points visible from miles away that shape the transhumance routes. A 120-metre arch doesn't go unnoticed by anyone moving through the area, and it's reasonable to assume Aloba has been a local landmark for far longer than it has been an international photo target.
International recognition of the Ennedi came late and all at once. In 2016, UNESCO inscribed the «Ennedi Massif: Natural and Cultural Landscape» on the World Heritage List as a mixed site, under cultural criterion (iii) and natural criteria (vii) and (ix), with a protected area of 2,441,200 hectares and a buffer zone of 777,800. Aloba lies inside the perimeter.
The following year, the Government of Chad signed a fifteen-year agreement with African Parks to create, fund and manage the Ennedi Natural and Cultural Reserve, covering more than 50,000 km², with management up and running from February 2018. The relationship went through a crisis in October 2025 —unilateral termination of the mandates on the 6th, reversed on the 17th— and was put back on track on 2 July 2026 with the signing of new co-management agreements covering Ennedi, Zakouma, Siniaka-Minia and the Aouk-Keïta landscape.
Meanwhile, Aloba became an image. The photo of the arch outlined against the sky, with the Milky Way crossing the span or a tiny human figure at the foot of a pillar, circulates through travel magazines, documentaries and social media, and —together with the Guelta d'Archei— it is Chad's visual calling card to the world. African Parks describes it in its own communications as one of the tallest and widest natural arches known and as a reminder of the need to protect the Ennedi's landscapes.
That is exactly the site's dilemma. The country needs tourism and is actively going after it: in February 2026 it relaunched the International Festival of Saharan Cultures, whose sixth edition was held from 7 to 13 February in Amdjarass, with an explicit message of sustainable tourism and South-South cooperation. But Western foreign ministries still formally advise against travel to the Ennedi, the logistics are still those of an expedition, and visitor numbers are counted in the hundreds per year.
In the very long run, the only certainty is geological: Aloba's lintel will keep thinning and one day it will fall, as every natural arch does. Until then, it's still there, never measured with instruments, with no trail and no sign, in the eastern corner of a massif three-quarters of which has yet to be explored.
