From above, the Aldabra atoll looks like a giant green fortress, adrift in the Indian Ocean. Approached at sea level it is more remarkable still, a protected paradise where nature has been left to form and crumble, grow, wither and regrow, undisturbed by human meddling.

Aldabra, a World Heritage site, consists of four coral islands encircling a large central lagoon. Although closest to Madagascar, it belongs to the Seychelles, 1,200km to the north-east. But, unlike the Seychelles, Aldabra does not welcome tourists; it is far off the shipping lanes and there is nowhere to stay. Even scientists, who are lured here by the world’s largest population of giant tortoises and the opportunity to study what happens to species when evolution is allowed to unfold unchecked, are only granted limited study licences.

The central lagoon is tidal: twice a day oceanic water whooshes in, bringing with it sharks, green turtles and vital nutrients. But when it drains, it leaves bizarre limestone structures—or champignons as they are known—teetering above the shallows like outspread parasols. Over time, the water erodes the coral, chipping away at the stem of the champignons until, eventually, the whole structure topples over.

Cheryl-Samantha Owen, a conservation scientist and photographer, was on an expedition to document the shark species when she took this picture of a five-metre-high champignon at low tide. “Everything you do on Aldabra is controlled by the tides,” she says. “You anchor off the reef and swim in through the channels, which can be pretty hairy.” Water flowing through the largest of the channels, Grande Passe, can run at seven knots on a spring tide. “The lagoon is an incredible green colour, which is reflected in the clouds above. It is an extraordinary place, full of mystique, a glimpse of what the world might be like if we hadn’t interfered."