In the centre of La Réunion, the tiny French island that bobs in the Indian Ocean between Madagascar and Mauritius, there is another sort of island. This one is even more isolated.

Formed when the magna chamber of a major volcano collapsed some three million years ago, the verdant Cirque de Mafate is separated from the rest of the island by sheer cliff walls, impressive mountains and thick tropical forest. The only way in or out of the amphitheatre-shaped valley is by foot – or, in a pinch, by helicopter.

Settlers arrived in the lush volcanic crater in the 18th Century. The first wave consisted of enslaved Africans fleeing their masters. Later came several impoverished French farmers whose plantations had failed after slavery was abolished. For generations, these settlers and their descendants were all but cut off from the outside world.

Today, Mafate’s roughly 800 inhabitants, known as Mafatais, live in tiny villages called îlets (a local Creole word that evokes the French word for tiny islands, îlots). Almost all of the residents are descendants of the original settlers, and each village consists of as few as two or three colourful, tin-roofed houses. There’s no electricity or water grid for the approximately 100sqkm valley. Doctors, police officers or foresters, if ever they are needed, are either brought in by helicopter or hike in.

In 2010, Unesco designated Mafate – as well as La Réunion’s two other, more accessible cirques and all of the island’s pitons and ramparts – as a World Heritage site, a move that’s brought in a recent influx of travellers. For now, those who visit this lost Garden of Eden are treated to breathtaking vistas, deep lush forests, wide African plains, wild rivers and a rich local culture – but that might not always be the case.

Much like the Galapagos Islands in the Pacific Ocean, La Réunion is home to a number of endemic birds, insects and plants not found anywhere else in the world. And Mafate is one of the few places left in the world to see an ecosystem that has developed over millions of years in relative isolation.

The valley is home to the Réunion stonechat, the Réunion marsh harrier, the Mascarene paradise flycatcher and the Réunion olive white-eye – all birds found only in the region. Several of the plants on view – most impressively the large highland tamarind tree that populates the Tamarin plains just below the Col des Bœufs mountain pass – grow nowhere else but Mafate.

Luckily – because it is the only option available to most visitors – hiking is an excellent way to experience the wild and isolated area, though the hundreds of kilometres of trails are not for the inexperienced or those suffering from vertigo, and they should never be hiked alone. The valley is accessible from a half-dozen trail heads, some leading over steep mountain passes, others along rivers and gullies, which lead to a network of shoulder-wide hiking trails that connect the hamlets like a national highway system.

Like the îlets, which are remarkably and charmingly different from one another, the trails never get boring. Awe-inspiring vistas quickly trade places with deep tropical forest or lush grassland. Knee-deep rivers are frequently crossed and serpentine paths are found only on the steepest of slopes.

On a hike between the breathtaking waterfall of Trois Roches and the southernmost îlet of Marla, I walked along a broad riverbed, climbed up an arid hillside, crossed another deeper, quicker-moving river, followed a gravel field so devoid of organic structure that the path was marked by kerns, and climbed into a subtropical moist broadleaf forest. I stared up 100m-high cliffs and peered down hills with kilometre-long views. On the two-and-a-half hour walk (signposts in Mafate describe time, rather than distance, between two points), I felt as though I had crossed in and out of half-a-dozen distinct and completely different geographical scenes.

And while the nature on display is magnificent, the people who live in the îlets’ many hand-decorated houses certainly add to the area’s draw. There’s an openness and directness that comes from needing to rely on the community and never having to lock a door. I was greeted warmly wherever I went, and conversations about the place, its history and – quite often – the neighbours came easily.

The epicentre of Mafate’s growing tourist trade is La Nouvelle, the valley’s biggest îlet, which has scores of well-maintained tin-roof houses, a church, a single-room school, a dispensary and a cemetery. Here, big yellow signs advertised sandwiches and hot and cold beverages, and music blared from some of the epiceries (small huts that sell pretty much everything). Yet despite the scores of hikers who were crisscrossing the village’s tiny dirt-trail grid, barefoot children ran up to ask me politely – and with genuine curiosity – where I’d come from and whether I needed directions.

It was both as though I was the first visitor to descend in days, and at the same time, just one of a growing number of outsiders bringing in business. I felt like I’d stepped back in time, while also looking into Mafate’s future.

If you liked this story, sign up for the weekly bbc.com features newsletter, called “If You Only Read 6 Things This Week”. A handpicked selection of stories from BBC Future, Earth, Culture, Capital, Travel and Autos, delivered to your inbox every Friday.