Somewhere north of Marseilles, not far from the white, chalk-cliffed banks of the River Ardèche, sits a farmhouse crammed with unseen treasures of 20th-century art. Quirky monster figures in bas-relief on the walls; intriguing paintings on doors; cupboard doors adorned with creatures that might be women or might be horses; an exquisite mosaic of a bat adorning the basement floor.

The work is by two of the biggest names in surrealism: any curator would give their right arm, or lobster telephone, or melting clock, to get in here. But they can’t: the house is not open to outsiders, and there are no plans to change that. More than 80 years after the artists who transformed this house closed its door for the final time, the place has pretty much remained in aspic; initially unknown to the art world, then ignored by it, and latterly a tantalizing but inaccessible trove.

The nearest village is called Saint-Martin-d’Ardèche, though the exact location of the house cannot be revealed as the owner fears a stampede of art lovers if he gives too much away. But he has been a fierce custodian of the work it contains, and he cares a great deal about the legacy of the artists who lived here, even though by the time his family moved in they were long gone and far away.

The pair had spent perhaps the most prolific and important period of their respective lives in this remote and idyllic spot, with hens clucking in the garden and vineyards laid out into the distance from the terrace. Neither of them would ever forget this place, and the months they spent here would inspire their work many decades later.

The artists in question were Max Ernst and Leonora Carrington, lovers when they lived here.

The house is not open to outsiders, and there are no plans to change that.

The house belonged to Leonora; she bought it with money sent from England. She had scandalized her Catholic family by eloping with Ernst, who was 26 years her senior, divorced and married to another woman, in the autumn of 1937. Though she turned her back on her relatives, and although the rift would never be healed, her mother secretly supported her, sending cash and food parcels as the war took hold and the south of France became ever further away from the family home in Lancashire.

I call her Leonora because she was my cousin. I was a member of that family she abandoned, but I found my way to her in 2006, and visited her regularly in the last five years of her life. She was in her eighties by the time I knocked on her door; my father, who had been five when she left, barely remembered her, and no one back home of my generation had ever met her.

She became my friend; more than my friend, my mentor. I sat with her at her kitchen table in Mexico City — because that was where she had ended up, after a story that was every bit as surreal as any of her paintings — and I learned many things. How to work out what matters most; how and why artists make their art; how to trust my instincts above all else. I learned about Leonora’s childhood, about the schools she had been expelled from, about the Renaissance artists of Florence who had opened her eyes to painting, about the school in London where she had painstakingly learned to draw, and about the moment when she smiled across a dinner table at a handsome, German artist many years her senior, who was as captivated with her as she was with him.

I learned about their love affair, and about how she realized early on that she had to make a clean break with her family, who not only didn’t approve of Ernst but whose concerns she found bourgeois and unhelpful. They were wealthy textile merchants but they knew little about art, and cared about it even less. When they told her they would cut her off without a penny she laughed in their faces, and hurried to catch the boat-train to Paris, where she joined Ernst and his friends — Picasso, Dalí, Duchamp, Breton, Méret Oppenheim — in St Germain, parachuted into place at a table in the most exciting spot in Western art in that moment, the café Les Deux Magots, unofficial HQ of the surrealist movement.

The months they spent here would inspire their work many decades later.

She and Ernst didn’t stay long in Paris; his wife was there as well, making the city too crowded. The pair of them — he styled himself the Bird Superior, and he called her his Bride of the Wind — travelled south to the Ardèche, where there would be just the two of them, and a world of artistic opportunities. At first they camped by the river, and then they bought the rundown farmhouse on the hill. They had some work done on it and moved in, paintbrushes and palettes in hand. Far from the whirl of Paris, they lapped up every minute of life in this backwater, with only one another and the birds and the trees, the vines and the church bells in the village below for company.

Their palettes are at the house to this day, propped up above a doorway; his board and hers, a silent memorial to the hours they spent here, painting and sculpting and creating, side by side. Sometimes they painted on canvases — she painted him, and he painted her — but mostly they painted the house. It was as though its very fabric became the vehicle through which they could express not only their individual art, but the art that connected them. And though their relationship would end, and they would move on, this house would remain as a witness to their idyllic sojourn.

Eighty years later I feel my visits here are a kind of homage to Leonora, who died aged 94 in 2011. I open the doors she opened; I admire the view she admired; I sit on the bed she slept in and I leaf through the books she was reading (their entire bookcase remains here, untouched: volumes of fairytales from England and Ireland brush up against German volumes that belonged to Ernst).

I muse on the art that festoons the place: the creamy white horse’s head affixed to the terrace wall, its nose turned to one side, its nostrils slightly flared, a quizzical look on its face. I pass the blood-red unicorn, its horn ahead of its fiery mane, painted in profile on the kitchen cupboard. I visit the bathroom to admire the elegant sphinx on the door panel, and like Marina Warner — who also knew Leonora, and who has also explored her work — I wonder whether this piece was part-executed by Leonor Fini, Max’s former lover, who at one point came to visit.

It’s difficult to know which pieces are by Leonora and which are by Ernst, and in many ways it doesn’t matter: this house represents their life together, the time when they were entwined, the time when they learned from one another. While Ernst was the senior in years, and while he was a far more established artist during their time together, theirs was a two-way street, and feminist art historians have spent the past few decades reframing the myth that a woman like Leonora, still only in her early twenties when she was living here, could only ever hope to be a muse, never a serious artist.

She joined Ernst and his friends — Picasso, Dalí, Duchamp, Breton, Méret Oppenheim — in St Germain, parachuted into place at a table.

Today her canvases sell for upwards of $3 million. Her novel The Hearing Trumpet has never been out of print, and over the past few months thousands of people have seen a major retrospective of her work, shown first in Copenhagen, then in Madrid. A couple of months ago an article in The Wall Street Journal mused on whether Leonora might be the new Frida Kahlo, noting the many parallels between their lives, including more famous (in their lifetimes) male partners (Diego Rivera was Kahlo’s husband), as well as major health crises — Kahlo never got over injuries sustained in a bus accident in her teens, and after leaving France, Leonora spent time incarcerated in a psychiatric institution, an experience from which she never fully recovered.

Leonora knew Kahlo briefly in Mexico City; she told me she remembered visiting the older artist, who was by then bedridden. She and I would laugh sometimes, over a cup of tea or a glass of tequila, about her work spawning — as Kahlo’s already had — an industry of mugs and fridge magnets, T-shirts and tote bags. It didn’t seem likely, back in those days, but today I have all these items and more. For a few surreal months last year her self-portrait greeted me at every London Tube station, the lead image in a Tate show, and in Italy the Venice Biennale was themed in her honor.

For my new book about Leonora I travelled across Europe, the US and Mexico in her footsteps: I followed her journey from Lancashire to London and Cornwall, and then to Paris and the Ardèche; to Santander and Madrid, and to Lisbon (where, in a thrilling twist, she would reconnect with Ernst, who was by that stage married to Peggy Guggenheim). I went to New York and Chicago to see the neighborhoods where she lived, and I returned to Mexico City, where the house she inhabited for 60 years is now a museum.

But of all these locations, every one crucial in her story, the place I felt her spirit closest was the house in France. Perhaps it was the art, perhaps it was the books, perhaps it was the way the whole place had been so miraculously preserved. Or perhaps it was the fact that this was where her journey as an artist truly started. In this house she learned from Ernst, and was able to experiment and practice and read and absorb ideas in a way she hadn’t before.

Ernst left first, involuntarily, in 1941: arrested as an enemy alien, he was taken into detention. Bereft and afraid, Leonora was persuaded by a friend to go to Spain. She left reluctantly, aware that if Max was released from prison this was where he would come to look for her, which is exactly what happened. Finding the house abandoned, he rescued the art he could carry and moved on: the owner of the house recalls that years later he returned again, and made casts of the sculptures and bas-relief works in the house.

Leonora, however, never came back. When I visited the house for the first time in the spring of 2011, I intended to take news of it back to her on my next trip to Mexico. But there was no time; she died a few weeks later. She had predicted, though, that I would find her old house exactly as she remembered it, art and all. She was right, and so it remains: a silent, mesmerizing, haunting tribute to two great artists, and the summer of their lives.

Joanna Moorhead is a U.K.-based journalist who has written for The Guardian, The Times of London, The Independent, and The Observer, among others