It’s 3 am in an outpost of Paradise, a wide, flat beach on Costa Rica’s Pacific coast. Under the cool, cream glow of the full moon, I am a spectator at a race for life.

At my feet, breaststroking along on tiny flippers over the hard sand at two yards a minute, is a survivor from the age of the dinosaurs no bigger than my hand. This leatherback turtle hatchling must reach the sea by dawn, or die. Just another of the stupendous struggles that play out nightly on Playa Grande. 

By day you would never suspect that this glorious, two mile long holiday beach is the site of one of the most important conservationprojects in the world. Bronzed surfers ride the crashing breakers. Winter tourists stroll. Pelicans spear vertically down into the clear blue sea.

But by night it stages one of nature’s great epics. Playa Grande is transformed into a high security turtle maternity ward. Guards from the environment ministry patrol. The public is excluded, and scientists take over, aided by volunteers from the Earthwatch conservation charity, supporting a cycle of life which may be millions of year old here.

After a few evening hours of sleep, I had joined Earthwatch volunteers Tami and Traci on the beach for the midnight shift. We wore T shirts with a conspicuous white turtle logo, to be easily checked by the guards. One of our duties was torelease three baby turtles that had hatched earlier in the day and missed the normal mass exodus to the sea.

We freed them from their bucket above high water. From then on our instructions were strict: “They are on their own.” They had to make the entire 200 yards to the sea unaided. It is essential that the hatchlings “taste” the beach, the dry sand , then the wet sand,  imprinting this information into their brains. Miraculously the females will recognise this exact beach  if they survive enormous wanderings and many perils and return here in 10 years or so to lay their eggs. 

It was astonishing to see. After turning a circle in the sand, all three set unerringly off towards the Pacific Ocean within seconds , as if drawn by a magnet. I edged alongside  “my” hatchling, clearly the strongest.  Tami’s  and Traci’s tagged along behind, keeping up a steady speed on their tiny flippers. It was enough for us just to be there. Left alone, the hatchlings would surely have fallen to the night heron, loitering with menace nearby. 

After over an hour my tiny companion had almost reached the first, four inches high, incoming wave. Twenty yards to go. Ten, five. Surely it couldn’t fail now?  For a worrying moment the hatchling veered sideways, perhaps attracted by the distant lights of Tamarindotown, two miles away.

Then the moon lit up a breaking wave, the turtle straightened and headed for the scintillating wall of water. The wave receded faster than I could walk, tugging the creature gently out to sea. A shower of meteorites sparkled overhead in celestial celebration.

Soon the other turtles made the waves. We wished them all well, like voyagers seen off on a hazardous space voyage. I hope that even now they are drifting serenely through the Pacific, chomping on jellyfish.  If they survive the ocean’s many perils, they might yet ready to return, around 2016, to spawn a new generation of leatherbacks.

I was in the company of some serious serial Earthwatch volunteers, working their way through a tantalizing list of world opportunities –current projects include helping leopards in Sri Laka, caring for coral in Thailand, aiding penguin research in S Africa. The aim is to give anyone the chance to help environmental projects, and have a good holiday at the same time.

I understood why this project is a top choice. The seven species of turtle are in peril round the world. They are hunted, their nestinghabitat is destroyed by development, and even when they mange to nest, their eggs are poached. The leatherback, biggest of all turtles, has survived for over 65 million years. They were alive when dinosaurs became extinct. But there are now fewer than 25,000 worldwide, and they face extinction in the Pacific.

In the 1990s,  Playa Grande, too, was heading for the brink as a turtle nesting area. Numbers slumped as almost all the eggs laid were taken to be sold as an alleged aphrodisiac. Then the Costa Rican government stepped in, and declared the beach a  nature reserve,setting its boundaries three miles  out to sea. An international team of scientists, headed by two of the planet’s leading turtle experts,began a conservation programme , with generous backing from writer John Grisham. They are applying the Churchillian dictum offighting on the beaches, to retake them for the turtles one by one. And it’s beginning to pay off.

I stayed with the scientists, from Australia, Spain the USA and S Africa, and the Earthwatch volunteers, in their comfortable base, a beachside house donated by a New York banker. We enjoy the conventional delights of Play Grande by day, snatched a hours sleep, then plunged into the dramas of the night.

We set off on at 11.30 pm with Eric, a US science graduate, to patrol a mile and a half of beach. We strode up and down, like guards in a strategic border, but looking for an welcome intruder. The leatheback is a leviathan, at  up to 8  feet long, and the weight of  sixmen. In the early 90s dozens emerged here every night out of the incoming tide in the four month breeding season. Now it is single figures.

For the first three hours we saw nothing. Then a message crackled through on Eric’s radio from the team patrolling the other half of the beach. “Turtle at 21” (the marker on the beach). She had laboured  up the sand, etching a deep track, since our last pass 30 minutesearlier.

We set off  at a quick march. And there she was, if anything more exciting than a lion or elephant sighting. They live on land, after all.It was as if a big, beautiful alien had landed, part of a sequence of birth and returning on this beach stretching back for aeons.

The turtle had found just the right spot and was scattering sand everywhere like an excited child on his or her first visit to the beach. She formed a shallow depression. Then, entirely by touch, she used her rear flippers to smooth out her egg chamber, like an expert potter shaping a pot from the inside.

After many minutes of  scouring and smoothing, she was satisfied. Then she began  to lay her eggs, with the delicacy of a top butler setting the best china on a table. Paul, one of the volunteers, gently moved her flipper aside so he could count them as they fell.

I helped Eric measure the turtle’s shell. Then he told me to pass a scanner over her back. Each turtle it is fitted, painlessly, with a tiny microchip. The scanner beeped, and I read out the number that flashed up on screen. It showed she was a returning mother, last seenhere three years ago. Then we left her to cover her eggs.

The scientists record every turtle egg laying  and hatching. By the time of my visit there had already been more nesting females than the whole of the previous season. Unlike the hatchlings, eaten by birds and killed by the sun, the turtles are safe in the beach.  Their problems are in their long years of drift through the Pacific. Too many are hooked by the dreaded long line fisheries, ironically designed to be safe to dolphins, set for tuna, snapper and swordfish, and drowned.

Local people now know they can actually make more money out of protecting the turtle than destroying it. Every night former egg poachers lead respectful groups of tourists long the beach. They pay $35 each and are shown one turtle laying eggs. Poaching is now be negligible.

I was back on Playa Grande, under a hot sun, just 12 hours later. It could have been any lightly used sweep of splendid holiday beach in the world. Sandpipers skittered daintily along the tide line. Frigate birds lumbered past out to sea. Ghost crabs scuttled past my toes  at high speed. The eggs from last night, safely buried above high tide, would lie undisturbed for two months,  before the hatchlings burst out of the sand and headed for the Pacific. The mothers were already far out to sea. Only a deep trail in the sand, as if from a primeval tractor,  reminded me that something very old and marvellous has passed this way.