Her ancestors had been doing this for a hundred million years: returning to the same isolated beaches to lay their eggs. Those ancestors had shared the ancient seas with long-necked plesiosaurs, fast-swimming ichthyosaurs and the terrifying mosasaurs, built like monumental crocodiles, with jaws capable of engulfing a fully grown leatherback turtle, whole.

She herself had made this journey a hundred times. Now she was tired. So tired. She would not come to this beach – or any other – ever again. And the knowledge that this was her last time gave extra urgency to the long, slow beating of her flippers.

She found the familiar gap in the reef and entered the calm lagoon. She surfaced and saw above her the great canopy of stars, like frozen fireworks. Soon she felt the sand on the underside of her shell, and began to drag herself up the beach. So graceful in the water, on land she was ungainly and awkward, moving as if carrying a great burden, and it was not long before she reached a point beyond exhaustion.

But she had to make it past the high-tide mark or her eggs would be swamped and lost; and so on she went. At last she found a place where the white coral sand was untouched by the waves, and she began to dig, pushing down with her back flippers and flicking the sand out in all directions, like a garden sprinkler.

It took her almost an hour, but finally she was content with the depth of the hole, and she began to lay. It took her another hour to fill her nest with the hundred and fifty eggs. When it was over, she pushed back the sand. It was still dark, but there was a slight lightening in the east as she hauled herself back to the warm sea.

The ache in her stomach told her that she hadn’t eaten for a long time, and she thought it would be good to swim through a swarm of jellyfish, munching away. Their stings couldn’t hurt her, and the soft bodies were easy on her old jaws. She moved serenely through the gap in the reef, her front flippers beating the water like leathery wings, and she sensed the bottom fall away to the dark, cold depths.

And then she felt something else. A movement, dark against the dark. Something huge, and deadly, and without pity.

She had fought sharks before. She knew where they were vulnerable. Her sharp, hooked beak could inflict serious damage on a shark’s gills. And her sheer bulk – she was two metres long and weighed more than 700 kg – made her a surprisingly formidable foe. And, besides, she was too tired to run. So she took a last gulp of air, and thinking perhaps of the tens of thousands of eggs she had laid, of the many thousands of young who had struggled down to the water, of the few who had made it through to adulthood, she dived to face the shadow as it rose.