Ostional is a four-mile stretch of black-sand beach in northwest Costa Rica. You probably haven’t heard of it – not many people have – but every year it plays host to one of the most dramatic, enormous, mind-blowing, jaw-dropping spectacles in nature.

Between June and December, for a couple of days synchronised with the phases of the moon, it is home to the greatest aggregation of reptiles on the planet.

They’re olive ridley turtles: giant ancient mariners, their heads encrusted with barnacles and shells, bearing the scars of shark bite and boat strike. They congregate at Ostional to breed and, soon after, the females come ashore to lay their eggs.

Unlike other species of turtle, they don’t sneak ashore silently and solo, hoping to evade the predators that would snatch their eggs and hatchlings. Instead, they arrive in multitudes so awe-inspiring that the predators are simply overwhelmed. Once, 500,000 turtles were counted coming ashore on this stretch of beach in a 72-hour period.

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The arribadas (arrivals) peak at night and at high tide. We arrived in mid-afternoon, at low tide, and one was already well under way.

Ostional is pretty impressive anyhow. Fearsome waves crash down on the black sand and, just beyond the break, jagged volcanic rocks send plumes of spray into the sky like geysers. Behind the beach is uninterrupted rain-forest, cloaking hills, then mountains. Now, though, all the eye could see was turtles: scrambling over each other’s shells in their desperate mission to find a spare yard of sand in which to dig their nest, battling up the gradient and colliding with their fellow reptiles heading back out to sea.

Behind them in the breakers, countless heads popped up from the foam, in a holding pattern, awaiting their turn.

Further up the beach, all that was visible off into the distance was a sea of wobbling boulders chucking sand up behind them. You couldn’t walk a pace without stumbling over a flipper. Huge gusts of sand were propelled forcibly into your pockets by their vigorous digging. You had to be attentive to prevent a disgruntled bill taking off your foot.

My guide in Costa Rica was Julio Rivera, naturalist and fixer extraordinaire. Julio is an amazing fellow: a mystical, magical Latin American character who left his family of 16 brothers and sisters at six years old in order to walk to the Caribbean “to find out if a negro’s skin was painted and would smear at the touch”. He gothis first pair of shoes at age 15. As a young man, he “died” after being bitten by a ferde-lance snake, and was brought back to life three days later, convinced he had seen God. After that, he did not cut his hair for a decade.

Julio knows every plant, tree and creature in this, the most biodiverse country in the world, and his wife has nicknamed him “the Queen”, as his travels are interrupted by continual waving – everyone in the country seems to be a personal friend. Julio was responsible for getting me here with such perfect timing, to see an estimated 75,000 turtles come ashore.

Within a few square feet, you can see every part of the process taking place. One turtle digs with her broad fore flippers; another turns to scoop out a deep well with her rear flippers; behind the next, you could watch the mucus-covered ping-pong balls plopping into the hole, then a turtle covering them, and another waddling her heavy carapace over the sand to flatten it. You could even see matchbox-sized hatchlings flapping for the sea.

You couldn’t take a single step without killing potential progeny. Mind you, that was a drop in the ocean. First, with so many turtles nesting in one place, single nesting sites can get turned over as many as 30 times – one mother lays her eggs, then another comes in and digs them up while preparing her own nest (the eggs all rot and perish).

More dramatic still is the predation. Flocks of vultures, 100 strong, sit behind the mothers and peck up the eggs as they drop. Dogs, raccoons, coyotes, night herons and majestic roseate spoonbills dig up the nests and devour everything. As the hatchlings emerge from the sand, they have to run the gauntlet of every scavenger within a 50-mile radius, all wanting to make them into a light appetiser. The local villagers take half a million eggs every night from the arribadas, then ship them off to restaurants all over the country. This is considered to be a good thing – at least the collections are well monitored, and considered to be sustainable.

Fewer than one in 1,000 youngsters will actually make it to maturity. It is enough. Here on Ostional beach, while the predators sit too bloated to move, a few hatchlings succeed in that final scamper into the surf and make their first frantic splashes on the road to majestic adulthood.

Travel details: Last Frontiers (01296 653000, www.lastfrontiers.com) has 14 nights in Costa Rica, with five near Ostional, from £2,200pp, B&B. The price includes flights from Heathrow to San Jose (via Madrid) with Iberia and private transfers.

Other operators to try include Geodyssey (020 7281 7788, www.geodyssey.co.uk) and Wildlife Worldwide (0845 130 6982, www.wildlifeworldwide.com).

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