Twenty miles or so east of Amsterdam, set between the new towns of Almere and Lelystad, and lying five metres below sea-level, is the youngest wilderness I have ever seen. The Oostvaardersplassen is now a vast region of grassland, reed-bed, shallow lake and ragged forest, over 6,000 hectares in extent. Sea eagles and marsh harriers hunt its wide skies, spoonbills and avocets stalk its marshes, and vast herds of red deer, wild ponies and Heck cattle graze its savannah.

But 40 years ago, the Oostvaardersplassen was underwater. Its name, which unfolds into English as "the place through which ships sail on the way from Amsterdam to the East Indies", alludes to its former status as sea. The existence of the Oostvaardersplassen, like much of the coastal Netherlands, is due to the Dutch genius for hydro-engineering. In the 1930s the government began an audacious project to dyke off and dry out four massive areas of land to the east of Amsterdam, at the southerly end of the Zuiderzee. By 1968 three such "polders" had been created, with all of the reclaimed land earmarked in advance for industry, agriculture and conurbation.

Then came the oil crisis of 1973. The Dutch economy lapsed into recession. Development of the polders faltered. The area between Almere and Lelystad—which had never been fully drained—became home to egrets instead of factories. Sensing opportunity, a small but determined group of conservationists set out to strategise, proselytise and wheedle the Oostvaardersplassen into being as a nature reserve unlike any other: a slab of wild land in the heart of one of the world's most densely settled nations.

Among European naturalists, the Oostvaardersplassen is a fabled place: radical in its permissive vision of how nature might be managed, and inspirational in its scale. I first heard tell of it a decade ago; this year, at last, I visited it. I caught a slow train out of Amsterdam, clacking past miles of suburb, and then miles of farmland: field and furrow, maize-stand and tulip row, straight-edged and ultra-ordered.

Then, after half an hour, the train passed a high screen of poplars and suddenly there was only open space to the north and east, for we were on the southern edge of the Oostvaardersplassen. It was as if I had entered another country, or even another continent. Hundreds of wild ponies milled and galloped and grazed on open grassland. Starlings perched on the backs of some of them, like ox-peckers on zebras. Five great egrets, white as paper, stepped slowly towards me in a line. A flock of geese turned and curved in the air, while through them on a straight vector flew three swans. A shimmering heat-haze rose in ripples from the earth, and the whole thing seemed hallucinatory—a Serengeti mirage, thrilling in its strangeness and size.

My companion for the day was Hans Breeveld, who has worked at the Oostvaardersplassen for more than 30 years. Hans is passionate about his landscape, but unromantic. He first worked there during a botulism outbreak which killed more than 30,000 birds. "My job was to canoe around wearing rubber gloves and pull the bodies out of the water. If the birds weren't dead, we killed them. It took months."

Deep in the reserve, we met a herd of 400 or so Konik ponies, the stallions back-kicking and biting, the foals skittish and bolty. "These animals have no tagging, no branding, no vaccination, and definitely no names," said Hans, with satisfaction. Marsh harriers flapped and cruised overhead. Red-headed smews paddled, dipped. A flotilla of coots chelped companionably as they cruised down-dyke. White butterflies settled on scat. The landscape felt riotous with life.

Then we passed into a ghost forest of elders and leafless willows, dead from dehydration but their pale limbs still standing: as if a forest had been designed by Beckett and Giacometti. Overhead, a goshawk—the first wild one I had ever seen—lifted from the grass and glided away, scattering smaller birds as it flew. A sea eagle took flight from a willow, leaving the tree-top shivering, dipping off on its vast wings, its primary feathers stretched out like a swimmer's fingers.

Sea eagles came here of their own accord five years ago, moving down into the area from Scandinavia. They were charismatic proof of the conservation ethic of the Oostvaardersplassen: increase scale, reduce management inputs, resist species farming, avoid deliverables and goals, and let wild nature take its course as far as possible.

In The Netherlands, the reserve is both celebrated and deplored. "We've all been threatened with violence," said Hans, as we passed a pair of spoonbills prospecting for lunch. "Recently, there was a threat of kneecapping." He shrugged. The controversy surrounds the welfare of the ponies and cattle, some of which have to be culled to prevent their starvation during the winters. "People have compared the Oostvaardersplassen to a concentration camp," said Hans. He left the idiocy of the analogy unglossed.

Late in the day we drove along the sea wall that bounds its northern edge for six miles, and makes its existence possible. The Ijsselmeer stretched sunlit to the north, white yachts plunging on the waves. Inland, beyond the perimeter of the reserve, wind turbines spun and chopped. I looked across the Oostvaardersplassen, and thought of what the American novelist Wallace Stegner had said in his famous "Wilderness Letter" of 1960. "We simply need…wild country available to us, even if we never do more than drive to its edge and look in," Stegner had written. "For it can be a means of reassuring ourselves of our sanity as creatures—a part of the geography of hope."■