The scimitar-horned oryx has been brought back from extinction through captive breeding. Conservationists hope it could help slow the spread of the Sahara Desert.

When Marie Petretto and John Newby arrived in central Chad in the spring of 2012, they'd been sent with a mission to see this wide, open landscape as an antelope might see it.

The Ouadi Rimé-Ouadi Achim Faunal Reserve, a protected area on the edge of the Sahara, is bigger than the Republic of Ireland, with vast expanses of sun-baked drylands. New arrivals usually assume "it's just desert", says Petretto, a biologist and wildlife veterinarian, but as they moved through, their team of ecologists and conservationists documented its vast ranges of Sahelian grasslands and wooded gorges. "At first the desert and arid lands seem very flat and homogeneous," she says. "Once you start exploring, you realise how incredibly varied they are."

Petretto recalls close to 100 gazelles bounding past their truck and finding sparse clusters of acacia, some filled with vultures. Though ominous, these trees operate like "umbrellas on the beach", she says, allowing wildlife to rest in a landscape that can reach 50C (122F). River valleys, known as wadis, burst into life during dramatic seasonal rains around July, and hold onto this humidity through drier months, when they support food plants, such as wild bitter melon, and small shrubs where animals can hide.

All these signs gave them confidence that this could be the place for an audacious experiment to bring back a species that had vanished entirely from the wild in the 1980s. While the hottest months in Ouadi Achim would be deadly for most animals, the environment was ready-made for the scimitar-horned oryx, an antelope that stands more than 1m (3.3ft) tall at the shoulder, and is named for its long, elegant horns that reach backwards over its body like a curved sword.

The oryx has evolved to live healthily around the edges of Sahara – able to survive for months without water – but, by 2012, it lived only in zoos and reserves scattered across the planet. If successful, their plan to release the oryx would make it just the sixth-ever large mammal to have been declared extinct in the wild then to have been brought back via captive breeding, following the Przewalski's horse, European Bison, red wolf, Père David's deer and the Arabian oryx.

On a drizzly morning in mid-September, Petretto is near England's southern coast to meet the globally coordinated conservation network that looked after the scimitar-horned oryx for decades – including wildlife keepers, breeders and poaching patrols. In a rolling paddock nearby, five oryx mill around zebras and a handful of rhinos. In the morning mist, a pair of antelopes jerk up and clash their impressive horns – which can grow to 1.5m (5 ft). This determines who sits at the top of the herd's social hierarchy, say Petretto and Tania Gilbert, head of conservation science at Marwell Wildlife, the conservation charity that runs this wildlife park. It's nothing to worry about, says Gilbert, although one young male, Conor, "seems to think he can take the rhino". Faced with this weirdly confident antelope squaring up to him, the large male rhino, surprisingly, backs off.

This 400-acre estate, Marwell Hall, was inhabited by monks and aristocratic families for hundreds of years (and is rumoured to be haunted by one of Henry VIII's wives). In 1969, it was bought by John Knowles, a wealthy chicken farmer who assembled the country's first zoological collection specifically devised to breed endangered animals. Oryx – whose numbers had plunged from around one million to fewer than – were one of the first animals he bought.

Oryx don't mind England's cloud and rain, says Gilbert, although this green paddock – similar to nearby fields filled with sheep and horses – makes an unlikely base from which to save this supremely desert-adapted species. Evolving to survive around the edge of the Sahara, these antelope have wide hooves to navigate loose sand and white backs to reflect the sun. They are able to survive for months on the water they get from food, conserving water through a system of "adaptive hyperthermia", which makes them able to tolerate their body temperature rising to 46C (115F) without sweating.

Despite once roaming thousands of miles from the Nile to the Atlantic and on both the Sahara's northern and southern rims, scimitar-horned oryx were

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